Cleanup the global scope a bit:

- Remove unused FocusChanged typedef.
- Remove unused centerOfAttentionHeroTag.
- Modernize static functions for interacting with Scrollable by moving them
  into the Scrollable class.

examples/widgets/ensure_visible.dart

@@ -85,7 +85,7 @@ class EnsureVisibleAppState extends State<EnsureVisibleApp> {
       cardModel: cardModels[index],
       selected: index == selectedCardIndex,
       onTap: (BuildContext context) {
-        ensureWidgetIsVisible(context, duration: const Duration(milliseconds: 200))
+        Scrollable.ensureVisible(context, duration: const Duration(milliseconds: 200))
         .then((_) {
           setState(() { selectedCardIndex = index; });
         });

examples/widgets/overlay_geometry.dart

@@ -129,7 +129,7 @@ class OverlayGeometryAppState extends State<OverlayGeometryApp> {
       markers[MarkerType.topLeft] = box.localToGlobal(new Point(0.0, 0.0));
       final Size size = box.size;
       markers[MarkerType.bottomRight] = box.localToGlobal(new Point(size.width, size.height));
-      final ScrollableState scrollable = findScrollableAncestor(target.currentContext);
+      final ScrollableState scrollable = Scrollable.of(target.currentContext);
       markersScrollOffset = scrollable.scrollOffset;
     });
   }

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

@@ -194,10 +194,10 @@ class DragTarget<T> extends StatefulComponent {
   final DragTargetWillAccept<T> onWillAccept;
   final DragTargetAccept<T> onAccept;
 
-  DragTargetState<T> createState() => new DragTargetState<T>();
+  _DragTargetState<T> createState() => new _DragTargetState<T>();
 }
 
-class DragTargetState<T> extends State<DragTarget<T>> {
+class _DragTargetState<T> extends State<DragTarget<T>> {
   final List<T> _candidateData = new List<T>();
   final List<dynamic> _rejectedData = new List<dynamic>();
 
@@ -279,7 +279,7 @@ class _DragAvatar<T> {
   final Widget feedback;
   final Offset feedbackOffset;
 
-  DragTargetState _activeTarget;
+  _DragTargetState _activeTarget;
   bool _activeTargetWillAcceptDrop = false;
   Offset _lastOffset;
   OverlayEntry _entry;
@@ -299,7 +299,7 @@ class _DragAvatar<T> {
     _lastOffset = globalPosition - dragStartPoint;
     _entry.markNeedsBuild();
     HitTestResult result = WidgetFlutterBinding.instance.hitTest(globalPosition + feedbackOffset);
-    DragTargetState target = _getDragTarget(result.path);
+    _DragTargetState target = _getDragTarget(result.path);
     if (target == _activeTarget)
       return;
     if (_activeTarget != null)
@@ -308,13 +308,13 @@ class _DragAvatar<T> {
     _activeTargetWillAcceptDrop = _activeTarget != null && _activeTarget.didEnter(data);
   }
 
-  DragTargetState _getDragTarget(List<HitTestEntry> path) {
+  _DragTargetState _getDragTarget(List<HitTestEntry> path) {
     // Look for the RenderBox that corresponds to the hit target (the hit target
     // widget builds a RenderMetadata box for us for this purpose).
     for (HitTestEntry entry in path) {
       if (entry.target is RenderMetaData) {
         RenderMetaData renderMetaData = entry.target;
-        if (renderMetaData.metaData is DragTargetState)
+        if (renderMetaData.metaData is _DragTargetState)
           return renderMetaData.metaData;
       }
     }

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

@@ -4,8 +4,6 @@
 
 import 'framework.dart';
 
-typedef void FocusChanged(GlobalKey key);
-
 // _noFocusedScope is used by Focus to track the case where none of the Focus
 // component's subscopes (e.g. dialogs) are focused. This is distinct from the
 // focused scope being null, which means that we haven't yet decided which scope

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

@@ -335,7 +335,7 @@ enum _StateLifecycle {
 /// The signature of setState() methods.
 typedef void StateSetter(VoidCallback fn);
 
-/// The logic and internal state for a StatefulComponent.
+/// The logic and internal state for a [StatefulComponent].
 abstract class State<T extends StatefulComponent> {
   /// The current configuration (an instance of the corresponding
   /// StatefulComponent class).
@@ -439,13 +439,13 @@ abstract class State<T extends StatefulComponent> {
   }
 }
 
-abstract class ProxyComponent extends Widget {
-  const ProxyComponent({ Key key, this.child }) : super(key: key);
+abstract class _ProxyComponent extends Widget {
+  const _ProxyComponent({ Key key, this.child }) : super(key: key);
 
   final Widget child;
 }
 
-abstract class ParentDataWidget extends ProxyComponent {
+abstract class ParentDataWidget extends _ProxyComponent {
   const ParentDataWidget({ Key key, Widget child })
     : super(key: key, child: child);
 
@@ -460,7 +460,7 @@ abstract class ParentDataWidget extends ProxyComponent {
   void applyParentData(RenderObject renderObject);
 }
 
-abstract class InheritedWidget extends ProxyComponent {
+abstract class InheritedWidget extends _ProxyComponent {
   const InheritedWidget({ Key key, Widget child })
     : super(key: key, child: child);
 
@@ -1248,8 +1248,8 @@ class StatefulComponentElement<T extends StatefulComponent, U extends State<T>>
   }
 }
 
-abstract class ProxyElement<T extends ProxyComponent> extends ComponentElement<T> {
-  ProxyElement(T widget) : super(widget) {
+abstract class _ProxyElement<T extends _ProxyComponent> extends ComponentElement<T> {
+  _ProxyElement(T widget) : super(widget) {
     _builder = (BuildContext context) => this.widget.child;
   }
 
@@ -1267,7 +1267,7 @@ abstract class ProxyElement<T extends ProxyComponent> extends ComponentElement<T
   void notifyDescendants(T oldWidget);
 }
 
-class ParentDataElement extends ProxyElement<ParentDataWidget> {
+class ParentDataElement extends _ProxyElement<ParentDataWidget> {
   ParentDataElement(ParentDataWidget widget) : super(widget);
 
   void mount(Element parent, dynamic slot) {
@@ -1300,7 +1300,7 @@ class ParentDataElement extends ProxyElement<ParentDataWidget> {
 
 
 
-class InheritedElement extends ProxyElement<InheritedWidget> {
+class InheritedElement extends _ProxyElement<InheritedWidget> {
   InheritedElement(InheritedWidget widget) : super(widget);
 
   void notifyDescendants(InheritedWidget oldWidget) {

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

@@ -56,8 +56,6 @@ import 'transitions.dart';
 // should interpolate the inherited properties from their value at the source to
 // their value at the target. See: https://github.com/flutter/flutter/issues/213
 
-final Object centerOfAttentionHeroTag = new Object();
-
 class _HeroManifest {
   const _HeroManifest({
     this.key,

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

@@ -52,6 +52,55 @@ abstract class Scrollable extends StatefulComponent {
   final SnapOffsetCallback snapOffsetCallback;
   final double snapAlignmentOffset;
 
+  /// Returns the closest enclosing scrollable for the given context.
+  static ScrollableState of(BuildContext context) {
+    return context.ancestorStateOfType(ScrollableState);
+  }
+
+  /// Scrolls the closest enclosing scrollable to make the given context visible.
+  static Future ensureVisible(BuildContext context, { Duration duration, Curve curve }) {
+    assert(context.findRenderObject() is RenderBox);
+    // TODO(abarth): This function doesn't handle nested scrollable widgets.
+
+    ScrollableState scrollable = Scrollable.of(context);
+    if (scrollable == null)
+      return new Future.value();
+
+    RenderBox targetBox = context.findRenderObject();
+    assert(targetBox.attached);
+    Size targetSize = targetBox.size;
+
+    RenderBox scrollableBox = scrollable.context.findRenderObject();
+    assert(scrollableBox.attached);
+    Size scrollableSize = scrollableBox.size;
+
+    double scrollOffsetDelta;
+    switch (scrollable.config.scrollDirection) {
+      case ScrollDirection.vertical:
+        Point targetCenter = targetBox.localToGlobal(new Point(0.0, targetSize.height / 2.0));
+        Point scrollableCenter = scrollableBox.localToGlobal(new Point(0.0, scrollableSize.height / 2.0));
+        scrollOffsetDelta = targetCenter.y - scrollableCenter.y;
+        break;
+      case ScrollDirection.horizontal:
+        Point targetCenter = targetBox.localToGlobal(new Point(targetSize.width / 2.0, 0.0));
+        Point scrollableCenter = scrollableBox.localToGlobal(new Point(scrollableSize.width / 2.0, 0.0));
+        scrollOffsetDelta = targetCenter.x - scrollableCenter.x;
+        break;
+      case ScrollDirection.both:
+        assert(false); // See https://github.com/flutter/engine/issues/888
+        break;
+    }
+
+    ExtentScrollBehavior scrollBehavior = scrollable.scrollBehavior;
+    double scrollOffset = (scrollable.scrollOffset + scrollOffsetDelta)
+      .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
+
+    if (scrollOffset != scrollable.scrollOffset)
+      return scrollable.scrollTo(scrollOffset, duration: duration, curve: curve);
+
+    return new Future.value();
+  }
+
   ScrollableState createState();
 }
 
@@ -261,69 +310,12 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   }
 }
 
-ScrollableState findScrollableAncestor(BuildContext context) {
-  ScrollableState result;
-  context.visitAncestorElements((Element element) {
-    if (element is StatefulComponentElement) {
-      if (element.state is ScrollableState) {
-        result = element.state;
-        return false;
-      }
-    }
-    return true;
-  });
-  return result;
-}
-
 class ScrollNotification extends Notification {
   ScrollNotification(this.scrollable, this.position);
   final ScrollableState scrollable;
   final double position;
 }
 
-Future ensureWidgetIsVisible(BuildContext context, { Duration duration, Curve curve }) {
-  assert(context.findRenderObject() is RenderBox);
-  // TODO(abarth): This function doesn't handle nested scrollable widgets.
-
-  ScrollableState scrollable = findScrollableAncestor(context);
-  if (scrollable == null)
-    return new Future.value();
-
-  RenderBox targetBox = context.findRenderObject();
-  assert(targetBox.attached);
-  Size targetSize = targetBox.size;
-
-  RenderBox scrollableBox = scrollable.context.findRenderObject();
-  assert(scrollableBox.attached);
-  Size scrollableSize = scrollableBox.size;
-
-  double scrollOffsetDelta;
-  switch (scrollable.config.scrollDirection) {
-    case ScrollDirection.vertical:
-      Point targetCenter = targetBox.localToGlobal(new Point(0.0, targetSize.height / 2.0));
-      Point scrollableCenter = scrollableBox.localToGlobal(new Point(0.0, scrollableSize.height / 2.0));
-      scrollOffsetDelta = targetCenter.y - scrollableCenter.y;
-      break;
-    case ScrollDirection.horizontal:
-      Point targetCenter = targetBox.localToGlobal(new Point(targetSize.width / 2.0, 0.0));
-      Point scrollableCenter = scrollableBox.localToGlobal(new Point(scrollableSize.width / 2.0, 0.0));
-      scrollOffsetDelta = targetCenter.x - scrollableCenter.x;
-      break;
-    case ScrollDirection.both:
-      assert(false); // See https://github.com/flutter/engine/issues/888
-      break;
-  }
-
-  ExtentScrollBehavior scrollBehavior = scrollable.scrollBehavior;
-  double scrollOffset = (scrollable.scrollOffset + scrollOffsetDelta)
-    .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
-
-  if (scrollOffset != scrollable.scrollOffset)
-    return scrollable.scrollTo(scrollOffset, duration: duration, curve: curve);
-
-  return new Future.value();
-}
-
 /// A simple scrollable widget that has a single child. Use this component if
 /// you are not worried about offscreen widgets consuming resources.
 class ScrollableViewport extends Scrollable {

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

@@ -18,10 +18,10 @@ abstract class StatusTransitionComponent extends StatefulComponent {
 
   Widget build(BuildContext context);
 
-  StatusTransitionState createState() => new StatusTransitionState();
+  _StatusTransitionState createState() => new _StatusTransitionState();
 }
 
-class StatusTransitionState extends State<StatusTransitionComponent> {
+class _StatusTransitionState extends State<StatusTransitionComponent> {
   void initState() {
     super.initState();
     config.performance.addStatusListener(_performanceStatusChanged);