Teach OverlayEntry how to rebuild in place

This patch makes it so drag-and-drop doesn't thrash the overlay list.

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

@@ -224,9 +224,12 @@ class _DragAvatar {
   }
 
   void rebuild(BuildContext context) {
-    _entry?.remove();
+    if (_entry == null) {
       _entry = new OverlayEntry(child: _build(context));
       Navigator.of(context).overlay.insert(_entry);
+    } else {
+      _entry.child = _build(context);
+    }
   }
 
   DragTargetState _getDragTarget(List<HitTestEntry> path) {
@@ -251,6 +254,7 @@ class _DragAvatar {
     _activeTarget = null;
     _activeTargetWillAcceptDrop = false;
     _entry.remove();
+    _entry = null;
     if (onDragFinished != null)
       onDragFinished();
   }

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

@@ -7,19 +7,26 @@ import 'framework.dart';
 
 class OverlayEntry {
   OverlayEntry({
-    this.child,
+    Widget child,
     bool opaque: false
-  }) : _opaque = opaque;
+  }) : _child = child, _opaque = opaque;
 
-  final Widget child;
+  Widget get child => _child;
+  Widget _child;
+  void set child (Widget value) {
+    if (_child == value)
+      return;
+    _child = value;
+    _rebuild();
+  }
 
   bool get opaque => _opaque;
   bool _opaque;
-  void set opaque(bool value) {
+  void set opaque (bool value) {
     if (_opaque == value)
       return;
     _opaque = value;
-    _state?.setState(() {});
+    _rebuild();
   }
 
   OverlayState _state;
@@ -29,6 +36,10 @@ class OverlayEntry {
     _state?._remove(this);
     _state = null;
   }
+
+  void _rebuild() {
+    _state?.setState(() {});
+  }
 }
 
 class Overlay extends StatefulComponent {