Added backgroundImage to CircleAvatar.

In order to have an efficient way to display clipped avatars,
backgroundImage was added inside of the container's BoxDecoration.
Fixes #4964. This commit also fixes #4567 where the radius property
actually sets the diamater.

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
 import 'constants.dart';
@@ -24,7 +25,8 @@ class CircleAvatar extends StatelessWidget {
     Key key,
     this.child,
     this.backgroundColor,
-    this.radius: 40.0
+    this.backgroundImage,
+    this.radius: 20.0
   }) : super(key: key);
 
   /// The widget below this widget in the tree.
@@ -34,6 +36,10 @@ class CircleAvatar extends StatelessWidget {
   /// color will cause the avatar to animate to the new color.
   final Color backgroundColor;
 
+  /// The background image of the circle. Changing the background
+  /// image will cause the avatar to animate to the new image.
+  final ImageProvider backgroundImage;
+
   /// The size of the avatar. Changing the radius will cause the
   /// avatar to animate to the new size.
   final double radius;
@@ -44,11 +50,14 @@ class CircleAvatar extends StatelessWidget {
     final Color color = backgroundColor ?? theme.primaryColor;
 
     return new AnimatedContainer(
-      width: radius,
-      height: radius,
+      width: radius * 2.0,
+      height: radius * 2.0,
       duration: kThemeChangeDuration,
       decoration: new BoxDecoration(
         backgroundColor: color,
+        backgroundImage: backgroundImage != null ? new BackgroundImage(
+          image: backgroundImage
+        ) : null,
         shape: BoxShape.circle
       ),
       child: new Center(

packages/flutter/test/material/circle_avatar_test.dart

+// Copyright 2016 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 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('CircleAvatar test', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Center(
+        child: new CircleAvatar(
+          backgroundColor: Colors.blue[400],
+          radius: 50.0,
+          child: new Text('Z')
+        )
+      )
+    );
+
+    RenderBox box = tester.renderObject(find.byType(CircleAvatar));
+    expect(box.size.width, equals(100.0));
+    expect(box.size.height, equals(100.0));
+
+    expect(find.text('Z'), findsOneWidget);
+  });
+}