Improve error message when ChildView lacks a MediaQuery

Fixes #2662

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

@@ -4,8 +4,9 @@
 
 import 'package:flutter/rendering.dart';
 
-import 'media_query.dart';
+import 'debug.dart';
 import 'framework.dart';
+import 'media_query.dart';
 
 export 'package:flutter/rendering.dart' show ChildViewConnection;
 
@@ -16,7 +17,7 @@ class ChildView extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    assert(MediaQuery.of(context) != null);
+    assert(debugCheckHasMediaQuery(context));
     return new _ChildViewWidget(
       child: child,
       scale: MediaQuery.of(context).devicePixelRatio

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

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'framework.dart';
+import 'media_query.dart';
+
+bool debugCheckHasMediaQuery(BuildContext context) {
+  assert(() {
+    if (MediaQuery.of(context) == null) {
+      Element element = context;
+      throw new FlutterError(
+        'No MediaQuery widget found.\n'
+        '${element.widget.runtimeType} widgets require a MediaQuery widget ancestor.\n'
+        'The specific widget that could not find a MediaQuery ancestor was:\n'
+        '  ${element.widget}'
+        'The ownership chain for the affected widget is:\n'
+        '  ${element.debugGetOwnershipChain(10)}'
+      );
+    }
+    return true;
+  });
+  return true;
+}

packages/flutter/lib/widgets.dart

@@ -12,6 +12,7 @@ export 'src/widgets/banner.dart';
 export 'src/widgets/basic.dart';
 export 'src/widgets/binding.dart';
 export 'src/widgets/child_view.dart';
+export 'src/widgets/debug.dart';
 export 'src/widgets/dismissable.dart';
 export 'src/widgets/drag_target.dart';
 export 'src/widgets/editable.dart';