SizeObserver crusade: Dismissable

Refactor Dismissable to not need a SizeObserver and to generally tighten
up the code a bit.

packages/flutter/lib/src/animation/animation_controller.dart

@@ -87,7 +87,12 @@ class AnimationController extends Animation<double>
 
   /// The current value of the animation.
   ///
-  /// Setting this value stops the current animation.
+  /// Setting this value notifies all the listeners that the value
+  /// changed.
+  ///
+  /// Setting this value also stops the controller if it is currently
+  /// running; if this happens, it also notifies all the status
+  /// listeners.
   double get value => _value;
   double _value;
   void set value(double newValue) {

packages/flutter/lib/src/widgets/transitions.dart

@@ -79,7 +79,9 @@ class SlideTransition extends AnimatedComponent {
     Animation<FractionalOffset> position,
     this.transformHitTests: true,
     this.child
-  }) : position = position, super(key: key, animation: position);
+  }) : position = position, super(key: key, animation: position) {
+    assert(position != null);
+  }
 
   /// The animation that controls the position of the child.
   ///
@@ -87,7 +89,6 @@ class SlideTransition extends AnimatedComponent {
   /// be translated horizontally by width * dx and vertically by height * dy.
   final Animation<FractionalOffset> position;
 
-
   /// Whether hit testing should be affected by the slide animation.
   ///
   /// If false, hit testing will proceed as if the child was not translated at