Added asserts.

diff --git a/examples/flutter_gallery/lib/demo/calculator/logic.dart b/examples/flutter_gallery/lib/demo/calculator/logic.dart
index d08f829..f7616f8 100644
--- a/examples/flutter_gallery/lib/demo/calculator/logic.dart
+++ b/examples/flutter_gallery/lib/demo/calculator/logic.dart

@@ -83,6 +83,7 @@
       case Operation.Division:
         return '  \u00F7  ';
     }
+    assert(operation != null);
     return null;
   }
 }

diff --git a/examples/flutter_gallery/lib/demo/grid_list_demo.dart b/examples/flutter_gallery/lib/demo/grid_list_demo.dart
index b0e99ae..4ed8d8f 100644
--- a/examples/flutter_gallery/lib/demo/grid_list_demo.dart
+++ b/examples/flutter_gallery/lib/demo/grid_list_demo.dart

@@ -46,7 +46,7 @@
   }
 
   final Photo photo;
-  final GridDemoTileStyle tileStyle;
+final GridDemoTileStyle tileStyle;
   final BannerTapCallback onBannerTap; // User taps on the photo's header or footer.
 
   void showPhoto(BuildContext context) {
@@ -130,6 +130,7 @@
           child: image
         );
     }
+    assert(tileStyle != null);
     return null;
   }
 }

diff --git a/examples/stocks/lib/main.dart b/examples/stocks/lib/main.dart
index 6ab9336..5e7b238 100644
--- a/examples/stocks/lib/main.dart
+++ b/examples/stocks/lib/main.dart

@@ -75,6 +75,7 @@
           accentColor: Colors.redAccent[200]
         );
     }
+    assert(_configuration.stockMode != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/animation/animations.dart b/packages/flutter/lib/src/animation/animations.dart
index 537cee9..62490a6 100644
--- a/packages/flutter/lib/src/animation/animations.dart
+++ b/packages/flutter/lib/src/animation/animations.dart

@@ -287,7 +287,8 @@
       case AnimationStatus.completed: return AnimationStatus.dismissed;
       case AnimationStatus.dismissed: return AnimationStatus.completed;
     }
-    return null;    
+    assert(status != null);
+    return null;
   }
 
   @override

diff --git a/packages/flutter/lib/src/material/button.dart b/packages/flutter/lib/src/material/button.dart
index efc4c84..1757522 100644
--- a/packages/flutter/lib/src/material/button.dart
+++ b/packages/flutter/lib/src/material/button.dart

@@ -227,6 +227,7 @@
           return Colors.white30;
       }
     }
+    assert(_colorBrightness != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/material/dialog.dart b/packages/flutter/lib/src/material/dialog.dart
index 4feae02..61f1d0e 100644
--- a/packages/flutter/lib/src/material/dialog.dart
+++ b/packages/flutter/lib/src/material/dialog.dart

@@ -57,12 +57,14 @@
   final List<Widget> actions;
 
   Color _getColor(BuildContext context) {
-    switch (Theme.of(context).brightness) {
+    Brightness brightness = Theme.of(context).brightness;
+    switch (brightness) {
       case Brightness.light:
         return Colors.white;
       case Brightness.dark:
         return Colors.grey[800];
     }
+    assert(brightness != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/material/drawer_item.dart b/packages/flutter/lib/src/material/drawer_item.dart
index 4f48ed8..40f32f5 100644
--- a/packages/flutter/lib/src/material/drawer_item.dart
+++ b/packages/flutter/lib/src/material/drawer_item.dart

@@ -67,7 +67,8 @@
           return Colors.white30;
         return null; // use default icon theme colour unmodified
     }
-    return null;  
+    assert(themeData.brightness != null);
+    return null;
   }
 
   TextStyle _getTextStyle(ThemeData themeData) {

diff --git a/packages/flutter/lib/src/material/icon.dart b/packages/flutter/lib/src/material/icon.dart
index bc65bd1..bb63b26 100644
--- a/packages/flutter/lib/src/material/icon.dart
+++ b/packages/flutter/lib/src/material/icon.dart

@@ -65,7 +65,8 @@
       case Brightness.light:
         return Colors.black;
     }
-    return null;    
+    assert(brightness != null);
+    return null;
   }
 
   Color _getDefaultColor(BuildContext context) {

diff --git a/packages/flutter/lib/src/material/raised_button.dart b/packages/flutter/lib/src/material/raised_button.dart
index b01f048..b45daf3 100644
--- a/packages/flutter/lib/src/material/raised_button.dart
+++ b/packages/flutter/lib/src/material/raised_button.dart

@@ -98,12 +98,14 @@
     } else {
       if (disabledColor != null)
         return disabledColor;
+      Brightness brightness = Theme.of(context).brightness;
       switch (Theme.of(context).brightness) {
         case Brightness.light:
           return Colors.black12;
         case Brightness.dark:
           return Colors.white12;
       }
+      assert(brightness != null);
       return null;
     }
   }

diff --git a/packages/flutter/lib/src/material/text_selection.dart b/packages/flutter/lib/src/material/text_selection.dart
index f428c0f..fdeae03 100644
--- a/packages/flutter/lib/src/material/text_selection.dart
+++ b/packages/flutter/lib/src/material/text_selection.dart

@@ -186,5 +186,6 @@
         child: handle
       );
   }
+  assert(type != null);
   return null;
 }

diff --git a/packages/flutter/lib/src/painting/box_painter.dart b/packages/flutter/lib/src/painting/box_painter.dart
index 077a3c2..88dacc9 100644
--- a/packages/flutter/lib/src/painting/box_painter.dart
+++ b/packages/flutter/lib/src/painting/box_painter.dart

@@ -1353,6 +1353,7 @@
         double distance = (position - center).distance;
         return distance <= math.min(size.width, size.height) / 2.0;
     }
+    assert(shape != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/painting/text_painter.dart b/packages/flutter/lib/src/painting/text_painter.dart
index dd2408f..cdcaf82 100644
--- a/packages/flutter/lib/src/painting/text_painter.dart
+++ b/packages/flutter/lib/src/painting/text_painter.dart

@@ -125,6 +125,7 @@
       case TextBaseline.ideographic:
         return _paragraph.ideographicBaseline;
     }
+    assert(baseline != null);
     return null;
   }
 
@@ -212,6 +213,7 @@
             ?? _getOffsetFromUpstream(offset, caretPrototype)
             ?? emptyOffset;
     }
+    assert(position.affinity != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/rendering/list.dart b/packages/flutter/lib/src/rendering/list.dart
index a391ddc..6cba7fd 100644
--- a/packages/flutter/lib/src/rendering/list.dart
+++ b/packages/flutter/lib/src/rendering/list.dart

@@ -82,6 +82,7 @@
       case Axis.horizontal:
         return padding.horizontal;
     }
+    assert(mainAxis != null);
     return null;
   }
 
@@ -109,6 +110,8 @@
         assert(debugThrowIfNotCheckingIntrinsics());
         return 0.0;
     }
+    assert(mainAxis != null);
+    return null;
   }
 
   @override

diff --git a/packages/flutter/lib/src/rendering/viewport.dart b/packages/flutter/lib/src/rendering/viewport.dart
index 9aa28b1..b6a70d0 100644
--- a/packages/flutter/lib/src/rendering/viewport.dart
+++ b/packages/flutter/lib/src/rendering/viewport.dart

@@ -57,6 +57,7 @@
       case ViewportAnchor.end:
         return paintOffset + (containerSize - contentSize);
     }
+    assert(anchor != null);
     return null;
   }
 
@@ -109,6 +110,8 @@
       case Axis.vertical:
         return offset.dx == 0.0;
     }
+    assert(direction != null);
+    return null;
   }
 
   /// The offset at which to paint the child.

diff --git a/packages/flutter/lib/src/widgets/banner.dart b/packages/flutter/lib/src/widgets/banner.dart
index be87827..5f2dd73 100644
--- a/packages/flutter/lib/src/widgets/banner.dart
+++ b/packages/flutter/lib/src/widgets/banner.dart

@@ -90,6 +90,8 @@
       case BannerLocation.topLeft:
         return 0.0;
     }
+    assert(location != null);
+    return null;
   }
 
   double _translationY(double height) {
@@ -101,6 +103,8 @@
       case BannerLocation.topLeft:
         return 0.0;
     }
+    assert(location != null);
+    return null;
   }
 
   double get _rotation {
@@ -112,6 +116,7 @@
       case BannerLocation.topLeft:
         return -math.PI / 4.0;
     }
+    assert(location != null);
     return null;
   }
 }

diff --git a/packages/flutter/lib/src/widgets/lazy_block.dart b/packages/flutter/lib/src/widgets/lazy_block.dart
index 6b08e76..b7acaca 100644
--- a/packages/flutter/lib/src/widgets/lazy_block.dart
+++ b/packages/flutter/lib/src/widgets/lazy_block.dart

@@ -731,6 +731,8 @@
       case Axis.vertical:
         return size.height;
     }
+    assert(widget.mainAxis != null);
+    return null;
   }
 
   Offset _getMainAxisOffsetForSize(Size size) {
@@ -740,7 +742,8 @@
       case Axis.vertical:
         return new Offset(0.0, size.height);
     }
-    return null;    
+    assert(widget.mainAxis != null);
+    return null;
   }
 
   static RenderBox _getNextWithin(_RenderLazyBlock block, RenderBox child) {

diff --git a/packages/flutter/lib/src/widgets/pageable_list.dart b/packages/flutter/lib/src/widgets/pageable_list.dart
index 3e547f1..611f60f 100644
--- a/packages/flutter/lib/src/widgets/pageable_list.dart
+++ b/packages/flutter/lib/src/widgets/pageable_list.dart

@@ -224,6 +224,7 @@
       case Axis.vertical:
         return box.size.height;
     }
+    assert(config.scrollDirection != null);
     return null;
   }
 
@@ -249,6 +250,8 @@
       case ViewportAnchor.end:
         return (_itemCount - scrollIndex - 1) % itemCount;
     }
+    assert(config.scrollAnchor != null);
+    return null;
   }
 
   @override
@@ -325,6 +328,8 @@
       case PageableListFlingBehavior.stopAtNextPage:
         return _flingToAdjacentItem(scrollVelocity);
     }
+    assert(config.itemsSnapAlignment != null);
+    return null;
   }
 
   @override

diff --git a/packages/flutter/lib/src/widgets/scrollable.dart b/packages/flutter/lib/src/widgets/scrollable.dart
index 38fabce..889a93d 100644
--- a/packages/flutter/lib/src/widgets/scrollable.dart
+++ b/packages/flutter/lib/src/widgets/scrollable.dart

@@ -285,6 +285,7 @@
       case ViewportAnchor.end:
         return pixelOffset;
     }
+    assert(config.scrollAnchor != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/widgets/scrollable_list.dart b/packages/flutter/lib/src/widgets/scrollable_list.dart
index 875d133..88497d0 100644
--- a/packages/flutter/lib/src/widgets/scrollable_list.dart
+++ b/packages/flutter/lib/src/widgets/scrollable_list.dart

@@ -239,6 +239,7 @@
         }
         break;
     }
+    assert(mainAxis != null);
     return null;
   }
 

diff --git a/packages/flutter/lib/src/widgets/text_selection.dart b/packages/flutter/lib/src/widgets/text_selection.dart
index 06124af..37feefc 100644
--- a/packages/flutter/lib/src/widgets/text_selection.dart
+++ b/packages/flutter/lib/src/widgets/text_selection.dart

@@ -294,6 +294,7 @@
       case TextDirection.rtl:
         return rtlType;
     }
+    assert(endpoint.direction != null);
     return null;
   }
 }

diff --git a/packages/flutter/lib/src/widgets/virtual_viewport.dart b/packages/flutter/lib/src/widgets/virtual_viewport.dart
index 59985c8..67c9fc1 100644
--- a/packages/flutter/lib/src/widgets/virtual_viewport.dart
+++ b/packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -70,7 +70,8 @@
       case ViewportAnchor.end:
         return scrollOffset;
     }
-    return null;    
+    assert(renderObject.anchor != null);
+    return null;
   }
 
   /// Returns a two-dimensional representation of the scroll offset, accounting

diff --git a/packages/flutter/test/widget/render_object_widget_test.dart b/packages/flutter/test/widget/render_object_widget_test.dart
index b64a48d..0b73b99 100644
--- a/packages/flutter/test/widget/render_object_widget_test.dart
+++ b/packages/flutter/test/widget/render_object_widget_test.dart

@@ -23,12 +23,14 @@
   TestOrientedBox({ Key key, Widget child }) : super(key: key, child: child);
 
   Decoration _getDecoration(BuildContext context) {
-    switch (MediaQuery.of(context).orientation) {
+    Orientation orientation = MediaQuery.of(context).orientation;
+    switch (orientation) {
       case Orientation.landscape:
         return new BoxDecoration(backgroundColor: const Color(0xFF00FF00));
       case Orientation.portrait:
         return new BoxDecoration(backgroundColor: const Color(0xFF0000FF));
     }
+    assert(orientation != null);
     return null;
   }
 

diff --git a/packages/flutter_tools/lib/src/application_package.dart b/packages/flutter_tools/lib/src/application_package.dart
index 2810901..ae3d5df 100644
--- a/packages/flutter_tools/lib/src/application_package.dart
+++ b/packages/flutter_tools/lib/src/application_package.dart

@@ -134,6 +134,7 @@
     case TargetPlatform.linux_x64:
       return null;
   }
+  assert(platform != null);
   return null;
 }
 

diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index c599cba..3664066 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart

@@ -99,6 +99,7 @@
     case 'linux-x64':
       return TargetPlatform.linux_x64;
   }
+  assert(platform != null);
   return null;
 }