Add more debugging information to Widgets.

Also, fix comment mentioning syncConstructorArguments.

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

@@ -712,6 +712,11 @@ class DefaultTextStyle extends InheritedWidget {
   }
 
   bool updateShouldNotify(DefaultTextStyle old) => style != old.style;
+
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    '$style'.split('\n').forEach(description.add);
+  }
 }
 
 class Text extends StatelessComponent {
@@ -738,6 +743,13 @@ class Text extends StatelessComponent {
       text = new StyledTextSpan(combinedStyle, [text]);
     return new Paragraph(text: text);
   }
+
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('"$data"');
+    if (style != null)
+      '$style'.split('\n').forEach(description.add);
+  }
 }
 
 class Image extends LeafRenderObjectWidget {

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

@@ -182,10 +182,15 @@ abstract class Widget {
   Element createElement();
 
   String toString() {
-    if (key == null)
-      return '$runtimeType';
-    return '$runtimeType-$key';
-  }
+    final String name = key == null ? '$runtimeType' : '$runtimeType-$key';
+    final List<String> data = <String>[];
+    debugFillDescription(data);
+    if (data.isEmpty)
+      return 'name';
+    return 'name(${data.join("; ")})';
+   }
+
+  void debugFillDescription(List<String> description) { }
 }
 
 /// RenderObjectWidgets provide the configuration for [RenderObjectElement]s,
@@ -786,6 +791,8 @@ abstract class Element<T extends Widget> implements BuildContext {
       description.add('no depth');
     if (widget == null)
       description.add('no widget');
+    else
+      widget.debugFillDescription(description);
   }
 
   String toStringDeep([String prefixLineOne = '', String prefixOtherLines = '']) {

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

@@ -318,7 +318,7 @@ class ScrollableViewportState extends ScrollableState<ScrollableViewport> {
     });
   }
   void _updateScrollBehaviour() {
-    // if you don't call this from build() or syncConstructorArguments(), you must call it from setState().
+    // if you don't call this from build(), you must call it from setState().
     scrollTo(scrollBehavior.updateExtents(
       contentExtent: _childSize,
       containerExtent: _viewportSize,