Make UINode's key a String

sky/sdk/lib/widgets/ink_well.dart

Index: sky/sdk/lib/widgets/ink_well.dart
diff --git a/sky/sdk/lib/widgets/ink_well.dart b/sky/sdk/lib/widgets/ink_well.dart
index cc2b24743cfe4715c6e39d5bbbf421c28d43731c..063fe0e3e794b5053f970579bd1025eccd3192ca 100644
--- a/sky/sdk/lib/widgets/ink_well.dart
+++ b/sky/sdk/lib/widgets/ink_well.dart
@@ -100,10 +100,9 @@ class RenderInkWell extends RenderProxyBox {
 }
 
 class InkWell extends OneChildRenderObjectWrapper {
-  InkWell({ UINode child, Object key })
-    : super(child: child, key: key);
-
-  RenderInkWell get root { RenderInkWell result = super.root; return result; }
+  InkWell({ String key, UINode child })
+    : super(key: key, child: child);
 
+  RenderInkWell get root => super.root;
   RenderInkWell createNode() => new RenderInkWell();
 }