Add avatars to material chips

Fixes #464

examples/material_gallery/lib/chip_demo.dart

@@ -23,7 +23,11 @@ class _ChipDemoState extends State<ChipDemo> {
     List<Widget> chips = <Widget>[
       new Chip(
         label: new Text('Apple')
-      )
+      ),
+      new Chip(
+        avatar: new CircleAvatar(label: 'B'),
+        label: new Text('Blueberry')
+      ),
     ];
 
     if (_showBananas) {

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

@@ -7,6 +7,9 @@ import 'package:flutter/widgets.dart';
 import 'colors.dart';
 import 'icon.dart';
 
+const double _kChipHeight = 32.0;
+const double _kAvatarDiamater = _kChipHeight;
+
 const TextStyle _kLabelStyle = const TextStyle(
   inherit: false,
   fontSize: 13.0,
@@ -21,26 +24,39 @@ final ColorFilter _kIconColorFilter = new ColorFilter.mode(
 class Chip extends StatelessComponent {
   const Chip({
     Key key,
-    this.icon,
+    this.avatar,
     this.label,
     this.onDeleted
   }) : super(key: key);
 
-  final Widget icon;
+  final Widget avatar;
   final Widget label;
   final VoidCallback onDeleted;
 
   Widget build(BuildContext context) {
     final bool deletable = onDeleted != null;
+    double leftPadding = 12.0;
+    double rightPadding = 12.0;
+
+    List<Widget> children = <Widget>[];
+
+    if (avatar != null) {
+      leftPadding = 0.0;
+      children.add(new Container(
+        margin: const EdgeDims.only(right: 8.0),
+        width: _kAvatarDiamater,
+        height: _kAvatarDiamater,
+        child: avatar
+      ));
+    }
 
-    List<Widget> children = <Widget>[
-      new DefaultTextStyle(
-        style: _kLabelStyle,
-        child: label
-      )
-    ];
+    children.add(new DefaultTextStyle(
+      style: _kLabelStyle,
+      child: label
+    ));
 
     if (deletable) {
+      rightPadding = 0.0;
       children.add(new GestureDetector(
         onTap: onDeleted,
         child: new Container(
@@ -54,13 +70,9 @@ class Chip extends StatelessComponent {
       ));
     }
 
-    EdgeDims padding = deletable ?
-      new EdgeDims.only(left: 12.0) :
-      new EdgeDims.symmetric(horizontal: 12.0);
-
     return new Container(
-      height: 32.0,
-      padding: padding,
+      height: _kChipHeight,
+      padding: new EdgeDims.only(left: leftPadding, right: rightPadding),
       decoration: new BoxDecoration(
         backgroundColor: Colors.grey[300],
         borderRadius: 16.0