Make UINode's key a String

We need the key to be a String even though we claimed we could support any
Object. Also, clean up some style nits including shortening the |root| getters
on OneChildRenderNodeWrappers.

Fixes https://github.com/domokit/sky_sdk/issues/26.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1173293005.

examples/stocks2/lib/stock_arrow.dart

@@ -12,7 +12,7 @@ import 'package:sky/widgets/basic.dart';
 
 class StockArrow extends Component {
 
-  StockArrow({ Object key, this.percentChange }) : super(key: key);
+  StockArrow({ String key, this.percentChange }) : super(key: key);
 
   final double percentChange;
 

examples/stocks2/lib/stock_list.dart

@@ -11,7 +11,7 @@ import 'stock_row.dart';
 class Stocklist extends FixedHeightScrollable {
 
   Stocklist({
-    Object key,
+    String key,
     this.stocks,
     this.query
   }) : super(itemHeight: StockRow.kHeight, key: key);

examples/stocks2/lib/stock_menu.dart

@@ -10,7 +10,7 @@ import 'package:sky/framework/theme/view_configuration.dart';
 class StockMenu extends Component {
 
   StockMenu({
-    Object key,
+    String key,
     this.controller,
     this.autorefresh: false,
     this.onAutorefreshChanged

examples/widgets/spinning_mixed.dart

@@ -24,7 +24,7 @@ void addFlexChildSolidColor(RenderFlex parent, sky.Color backgroundColor, { int
 
 // Solid colour, Widget version
 class Rectangle extends Component {
-  Rectangle(this.color, { Object key }) : super(key: key);
+  Rectangle(this.color, { String key }) : super(key: key);
   final Color color;
   UINode build() {
     return new Flexible(