Include BoxDecoration description in Container toString

Have BoxDecoration default to one-line dumps

Style nits in a rendering example

examples/rendering/shadowed_box.dart

@@ -8,16 +8,19 @@ import 'package:flutter/rendering.dart';
 import 'package:flutter/material.dart';
 
 void main() {
-  var coloredBox = new RenderDecoratedBox(
+  RenderBox coloredBox = new RenderDecoratedBox(
     decoration: new BoxDecoration(
       gradient: new RadialGradient(
         center: Point.origin, radius: 500.0,
-        colors: <Color>[Colors.yellow[500], Colors.blue[500]]),
-      boxShadow: elevationToShadow[8])
+        colors: <Color>[Colors.yellow[500], Colors.blue[500]]
+      ),
+      boxShadow: elevationToShadow[8]
+    )
   );
-  var paddedBox = new RenderPadding(
+  RenderBox paddedBox = new RenderPadding(
     padding: const EdgeDims.all(50.0),
-    child: coloredBox);
+    child: coloredBox
+  );
   new RenderingFlutterBinding(root: new RenderDecoratedBox(
     decoration: const BoxDecoration(
       backgroundColor: const Color(0xFFFFFFFF)

packages/flutter/lib/src/painting/box_painter.dart

@@ -787,6 +787,9 @@ class BoxDecoration extends Decoration {
     );
   }
 
+  /// Stringifies the BoxDecoration. By default, the output will be on one line.
+  /// If the method is passed a non-empty string argument, then the output will
+  /// span multiple lines, each prefixed by that argument.
   String toString([String prefix = '']) {
     List<String> result = <String>[];
     if (backgroundColor != null)
@@ -803,6 +806,8 @@ class BoxDecoration extends Decoration {
       result.add('${prefix}gradient: $gradient');
     if (shape != BoxShape.rectangle)
       result.add('${prefix}shape: $shape');
+    if (prefix == '')
+      return '$runtimeType(${result.join(', ')})';
     if (result.isEmpty)
       return '$prefix<no decorations specified>';
     return result.join('\n');

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

@@ -908,9 +908,9 @@ class Container extends StatelessComponent {
     if (constraints != null)
       description.add('$constraints');
     if (decoration != null)
-      description.add('has background');
+      description.add('bg: $decoration');
     if (foregroundDecoration != null)
-      description.add('has foreground');
+      description.add('fg: $foregroundDecoration');
     if (margin != null)
       description.add('margin: $margin');
     if (padding != null)