Add asserts to catch potential misuses of the rendering framework.

New asserts:
- verify that after layout, the size fits the constraints
- verify that after layout, the size isn't infinite
- verify that you don't set the size in performLayout() if you have
  sizedByParent set
- verify that nobody reads your size during layout except you, or your
  parent if they said parentUsesSize:true

Fixes some bugs found by those asserts:
- RenderBlock, RenderStack, and RenderScaffold were not always setting
  parentUsesSize correctly
- RenderScaffold was setting its slot entries to null rather than
  removing them when the slot went away, which led to null derefs in
  certain circumstances

Also, rename a local variable in RenderStack.performLayout() because
it was shadowing a variable on the object itself, which was really
confusing when I first tried to debug this function...

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1213473003.

examples/rendering/sector_layout.dart

diff --git a/examples/rendering/sector_layout.dart b/examples/rendering/sector_layout.dart
index b17b31b..85dcc41 100644
--- a/examples/rendering/sector_layout.dart
+++ b/examples/rendering/sector_layout.dart
@@ -75,6 +75,17 @@
   }
 
   SectorConstraints get constraints => super.constraints;
+  bool debugDoesMeetConstraints() {
+    assert(constraints != null);
+    assert(deltaRadius != null);
+    assert(deltaRadius < double.INFINITY);
+    assert(deltaTheta != null);
+    assert(deltaTheta < double.INFINITY);
+    return constraints.minDeltaRadius <= deltaRadius &&
+           deltaRadius <= math.max(constraints.minDeltaRadius, constraints.maxDeltaRadius) &&
+           constraints.minDeltaTheta <= deltaTheta &&
+           deltaTheta <= math.max(constraints.minDeltaTheta, constraints.maxDeltaTheta);
+  }
   void performResize() {
     // default behaviour for subclasses that have sizedByParent = true
     deltaRadius = constraints.constrainDeltaRadius(0.0);