Resurrect spinning_mixed.

It had bit-rotted. Also improve our error reporting for missing Material.

examples/widgets/spinning_mixed.dart

@@ -40,10 +40,10 @@ void attachWidgetTreeToRenderTree(RenderProxyBox container) {
       child: new Column(
         children: <Widget>[
           new Rectangle(const Color(0xFF00FFFF)),
-          new Container(
+          new Material(
+            child: new Container(
               padding: new EdgeDims.all(10.0),
               margin: new EdgeDims.all(10.0),
-            decoration: new BoxDecoration(backgroundColor: const Color(0xFFCCCCCC)),
               child: new Row(
                 children: <Widget>[
                   new RaisedButton(
@@ -62,6 +62,7 @@ void attachWidgetTreeToRenderTree(RenderProxyBox container) {
                 ],
                 justifyContent: FlexJustifyContent.spaceAround
               )
+            )
           ),
           new Rectangle(const Color(0xFFFFFF00)),
         ],
@@ -86,6 +87,7 @@ void rotate(Duration timeStamp) {
 }
 
 void main() {
+  WidgetFlutterBinding.ensureInitialized();
   RenderProxyBox proxy = new RenderProxyBox();
   attachWidgetTreeToRenderTree(proxy);
 
@@ -97,7 +99,6 @@ void main() {
   transformBox = new RenderTransform(child: flexRoot, transform: new Matrix4.identity());
   RenderPadding root = new RenderPadding(padding: new EdgeDims.all(80.0), child: transformBox);
 
-  WidgetFlutterBinding.ensureInitialized()
-   ..renderView.child = root
-   ..addPersistentFrameCallback(rotate);
+  WidgetFlutterBinding.instance.renderView.child = root;
+  WidgetFlutterBinding.instance.addPersistentFrameCallback(rotate);
 }

packages/flutter/lib/src/material/debug.dart

@@ -8,11 +8,14 @@ import 'material.dart';
 
 bool debugCheckHasMaterial(BuildContext context) {
   assert(() {
-    if (context.widget is Material || context.ancestorWidgetOfExactType(Material) != null)
-      return true;
+    if (context.widget is! Material && context.ancestorWidgetOfExactType(Material) == null) {
       Element element = context;
-    debugPrint('${context.widget} needs to be placed inside a Material widget. Ownership chain:\n${element.debugGetOwnershipChain(10)}');
-    return false;
+      throw new WidgetError(
+        'Missing Material widget.',
+        '${context.widget} needs to be placed inside a Material widget. Ownership chain:\n${element.debugGetOwnershipChain(10)}'
+      );
+    }
+    return true;
   });
   return true;
 }