Don't register gesture detectors when Scrollable can't scroll

Previously, if you used a number of nested Blocks, they'd each try to register
drag gesture detectors even though they can't actually scroll. This CL teaches
Scrollable to watch for drag gestures only when it can actually scroll.

sky/packages/sky/lib/src/animation/scroll_behavior.dart

diff --git a/sky/packages/sky/lib/src/animation/scroll_behavior.dart b/sky/packages/sky/lib/src/animation/scroll_behavior.dart
index cd20976..14d47b0 100644
--- a/sky/packages/sky/lib/src/animation/scroll_behavior.dart
+++ b/sky/packages/sky/lib/src/animation/scroll_behavior.dart
@@ -19,6 +19,9 @@
 
   /// The new scroll offset to use when the user attempts to scroll from the given offset by the given delta
   double applyCurve(double scrollOffset, double scrollDelta);
+
+  /// Whether this scroll behavior currently permits scrolling
+  bool get isScrollable => true;
 }
 
 /// A scroll behavior for a scrollable widget with linear extent

sky/packages/sky/lib/src/widgets/scrollable.dart

diff --git a/sky/packages/sky/lib/src/widgets/scrollable.dart b/sky/packages/sky/lib/src/widgets/scrollable.dart
index 2fb8efa..2617f35 100644
--- a/sky/packages/sky/lib/src/widgets/scrollable.dart
+++ b/sky/packages/sky/lib/src/widgets/scrollable.dart
@@ -9,6 +9,7 @@
 import 'package:newton/newton.dart';
 import 'package:sky/animation.dart';
 import 'package:sky/gestures/constants.dart';
+import 'package:sky/gestures/scroll.dart';
 import 'package:sky/src/rendering/box.dart';
 import 'package:sky/src/rendering/viewport.dart';
 import 'package:sky/src/widgets/basic.dart';
@@ -79,13 +80,25 @@
 
   Widget buildContent();
 
+  GestureDragUpdateCallback _getDragUpdateHandler(ScrollDirection direction) {
+    if (scrollDirection != direction || !scrollBehavior.isScrollable)
+      return null;
+    return scrollBy;
+  }
+
+  GestureDragEndCallback _getDragEndHandler(ScrollDirection direction) {
+    if (scrollDirection != direction || !scrollBehavior.isScrollable)
+      return null;
+    return _maybeSettleScrollOffset;
+  }
+
   Widget build() {
     return new GestureDetector(
-      onVerticalDragUpdate: scrollDirection == ScrollDirection.vertical ? scrollBy : null,
-      onVerticalDragEnd: scrollDirection == ScrollDirection.vertical ? _maybeSettleScrollOffset : null,
-      onHorizontalDragUpdate: scrollDirection == ScrollDirection.horizontal ? scrollBy : null,
-      onHorizontalDragEnd: scrollDirection == ScrollDirection.horizontal ? _maybeSettleScrollOffset : null,
-      onFling: _handleFling,
+      onVerticalDragUpdate: _getDragUpdateHandler(ScrollDirection.vertical),
+      onVerticalDragEnd: _getDragEndHandler(ScrollDirection.vertical),
+      onHorizontalDragUpdate: _getDragUpdateHandler(ScrollDirection.horizontal),
+      onHorizontalDragEnd: _getDragEndHandler(ScrollDirection.horizontal),
+      onFling: scrollBehavior.isScrollable ? _handleFling : null,
       child: new Listener(
         child: buildContent(),
         onPointerDown: _handlePointerDown,