Change the default MaterialType to canvas

Canvas is by far the most popular type of material by call site (and makes for
a faster default because it doesn't have a RRect clip).

examples/demo_launcher/lib/main.dart

@@ -193,7 +193,6 @@ class DemoHome extends StatelessComponent {
     return new Scaffold(
       toolBar: new ToolBar(center: new Text('Sky Demos')),
       body: new Material(
-        type: MaterialType.canvas,
         child: new DemoList()
       )
     );

examples/fitness/lib/feed.dart

@@ -15,7 +15,6 @@ class FitnessItemList extends StatelessComponent {
 
   Widget build(BuildContext context) {
     return new Material(
-      type: MaterialType.canvas,
       child: new ScrollableList<FitnessItem>(
         padding: const EdgeDims.all(4.0),
         items: items,
@@ -172,10 +171,9 @@ class FeedFragmentState extends State<FeedFragment> {
   Widget buildBody() {
     TextStyle style = Theme.of(context).text.title;
     if (config.userData == null)
-      return new Material(type: MaterialType.canvas);
+      return new Material();
     if (config.userData.items.length == 0)
       return new Material(
-        type: MaterialType.canvas,
         child: new Row(
           [new Text("No data yet.\nAdd some!", style: style)],
           justifyContent: FlexJustifyContent.center
@@ -189,7 +187,6 @@ class FeedFragmentState extends State<FeedFragment> {
         );
       case FitnessMode.chart:
         return new Material(
-          type: MaterialType.canvas,
           child: new Container(
             padding: const EdgeDims.all(20.0),
             child: buildChart()

examples/fitness/lib/meal.dart

@@ -86,7 +86,6 @@ class MealFragmentState extends State<MealFragment> {
   Widget buildBody() {
     Meal meal = new Meal(when: new DateTime.now());
     return new Material(
-      type: MaterialType.canvas,
       child: new ScrollableViewport(
         child: new Container(
           padding: const EdgeDims.all(20.0),

examples/fitness/lib/measurement.dart

@@ -173,7 +173,6 @@ class MeasurementFragmentState extends State<MeasurementFragment> {
     Measurement measurement = new Measurement(when: _when);
     // TODO(jackson): Revisit the layout of this pane to be more maintainable
     return new Material(
-      type: MaterialType.canvas,
       child: new Container(
         padding: const EdgeDims.all(20.0),
         child: new Column([

examples/fitness/lib/settings.dart

@@ -93,7 +93,6 @@ class SettingsFragmentState extends State<SettingsFragment> {
 
   Widget buildSettingsPane(BuildContext context) {
     return new Material(
-      type: MaterialType.canvas,
       child: new ScrollableViewport(
         child: new Container(
           padding: const EdgeDims.symmetric(vertical: 20.0),

examples/stocks/lib/stock_list.dart

@@ -13,7 +13,6 @@ class StockList extends StatelessComponent {
 
   Widget build(BuildContext context) {
     return new Material(
-      type: MaterialType.canvas,
       child: new ScrollableList<Stock>(
         items: stocks,
         itemExtent: StockRow.kHeight,

examples/stocks/lib/stock_settings.dart

@@ -84,7 +84,6 @@ class StockSettingsState extends State<StockSettings> {
     // TODO(ianh): Once we have the gesture API hooked up, fix https://github.com/domokit/mojo/issues/281
     // (whereby tapping the widgets below causes both the widget and the menu item to fire their callbacks)
     return new Material(
-      type: MaterialType.canvas,
       child: new ScrollableViewport(
         child: new Container(
           padding: const EdgeDims.symmetric(vertical: 20.0),

examples/stocks/lib/stock_symbol_viewer.dart

@@ -33,7 +33,6 @@ class StockSymbolViewerState extends State<StockSymbolViewer> {
         center: new Text('${config.stock.name} (${config.stock.symbol})')
       ),
       body: new Material(
-        type: MaterialType.canvas,
         child: new Block([
           new Container(
             padding: new EdgeDims.all(20.0),

examples/widgets/scale.dart

@@ -61,7 +61,6 @@ class ScaleAppState extends State<ScaleApp> {
         toolBar: new ToolBar(
             center: new Text('Scale Demo')),
         body: new Material(
-          type: MaterialType.canvas,
           child: new GestureDetector(
             onScaleStart: _handleScaleStart,
             onScaleUpdate: _handleScaleUpdate,

examples/widgets/sector.dart

@@ -65,7 +65,6 @@ class SectorApp extends MaterialApp {
 
   Widget buildBody() {
     return new Material(
-      type: MaterialType.canvas,
       child: new Column([
           new Container(
             padding: new EdgeDims.symmetric(horizontal: 8.0, vertical: 25.0),

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

@@ -118,6 +118,7 @@ class Dialog extends StatelessComponent {
             child: new Material(
               level: 4,
               color: _getColor(context),
+              type: MaterialType.card,
               child: new IntrinsicWidth(
                 child: new Block(dialogBody)
               )

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

@@ -23,7 +23,7 @@ class Material extends StatelessComponent {
   Material({
     Key key,
     this.child,
-    this.type: MaterialType.card,
+    this.type: MaterialType.canvas,
     this.level: 0,
     this.color,
     this.textStyle

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

@@ -79,7 +79,6 @@ class SnackBar extends StatelessComponent {
           child: new Material(
             level: 2,
             color: _kSnackBackground,
-            type: MaterialType.canvas,
             child: new Container(
               margin: const EdgeDims.symmetric(horizontal: _kSideMargins),
               child: new DefaultTextStyle(