diff --git a/sky/sdk/lib/editing/input.dart b/sky/sdk/lib/editing/input.dart
index 91eb5e316b72d..0f0e3e5217488 100644
--- a/sky/sdk/lib/editing/input.dart
+++ b/sky/sdk/lib/editing/input.dart
@@ -19,7 +19,7 @@ const EdgeDims _kTextfieldPadding = const EdgeDims.symmetric(vertical: 8.0);
 class Input extends StatefulComponent {
 
   Input({
-    Key key,
+    GlobalKey key,
     this.placeholder,
     this.onChanged
   }): super(key: key);
diff --git a/sky/sdk/lib/widgets/basic.dart b/sky/sdk/lib/widgets/basic.dart
index 5e7ba5dee04b7..f7df35c8f5aa8 100644
--- a/sky/sdk/lib/widgets/basic.dart
+++ b/sky/sdk/lib/widgets/basic.dart
@@ -22,7 +22,7 @@ import 'package:sky/widgets/widget.dart';
 export 'package:sky/rendering/box.dart' show BackgroundImage, BoxConstraints, BoxDecoration, Border, BorderSide, EdgeDims;
 export 'package:sky/rendering/flex.dart' show FlexDirection, FlexJustifyContent, FlexAlignItems;
 export 'package:sky/rendering/object.dart' show Point, Offset, Size, Rect, Color, Paint, Path;
-export 'package:sky/widgets/widget.dart' show Key, Widget, Component, StatefulComponent, App, runApp, Listener, ParentDataNode;
+export 'package:sky/widgets/widget.dart' show Key, GlobalKey, Widget, Component, StatefulComponent, App, runApp, Listener, ParentDataNode;
 
 
 // PAINTING NODES
diff --git a/sky/sdk/lib/widgets/focus.dart b/sky/sdk/lib/widgets/focus.dart
index 5abb6fd1b4377..c50d434d622fc 100644
--- a/sky/sdk/lib/widgets/focus.dart
+++ b/sky/sdk/lib/widgets/focus.dart
@@ -14,11 +14,11 @@ class Focus extends Inherited {
 
   Focus({
     GlobalKey key,
-    GlobalKey initialFocus,
+    this.initialFocus,
     Widget child
   }) : super(key: key, child: child);
 
-  GlobalKey initialFocus;
+  final GlobalKey initialFocus;
 
   GlobalKey _currentlyFocusedKey;
   GlobalKey get currentlyFocusedKey {
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index 83b04d002a4ac..a7de0ca712bd5 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -140,13 +140,46 @@ abstract class Widget {
       _notifyingMountStatus = false;
       sky.tracing.end("Widget._notifyMountStatusChanged");
     }
+    assert(_debugDuplicateGlobalKeys.isEmpty);
   }
 
+  static final Map<GlobalKey, Widget> _globalKeys = new Map<GlobalKey, Widget>();
+  static final Map<GlobalKey, int> _debugDuplicateGlobalKeys = new Map<GlobalKey, int>();
+
   /// Override this function to learn when this [Widget] enters the widget tree.
-  void didMount() { }
+  void didMount() {
+    if (key is GlobalKey) {
+      assert(() {
+        if (_globalKeys.containsKey(key)) {
+          int oldCount = _debugDuplicateGlobalKeys.putIfAbsent(key, () => 1);
+          assert(oldCount >= 1);
+          _debugDuplicateGlobalKeys[key] = oldCount + 1;
+        }
+        return true;
+      });
+      _globalKeys[key] = this;
+    }
+  }
 
   /// Override this function to learn when this [Widget] leaves the widget tree.
-  void didUnmount() { }
+  void didUnmount() {
+    if (key is GlobalKey) {
+      assert(() {
+        if (_globalKeys.containsKey(key) && _debugDuplicateGlobalKeys.containsKey(key)) {
+          int oldCount = _debugDuplicateGlobalKeys[key];
+          assert(oldCount >= 2);
+          if (oldCount == 2) {
+            _debugDuplicateGlobalKeys.remove(key);
+          } else {
+            _debugDuplicateGlobalKeys[key] = oldCount - 1;
+          }
+        }
+        return true;
+      });
+      if (_globalKeys[key] == this)
+        _globalKeys.remove(key);
+    }
+  }
 
   RenderObject _root;