SnackBar should have a single optional action

The example in the spec with multiple actions is an anti-example.

Fixes #1876

examples/fitness/lib/feed.dart

@@ -110,11 +110,12 @@ class FeedFragmentState extends State<FeedFragment> {
     config.onItemDeleted(item);
     Scaffold.of(context).showSnackBar(new SnackBar(
       content: new Text("Item deleted."),
-      actions: <SnackBarAction>[
-        new SnackBarAction(label: "UNDO", onPressed: () {
+      action: new SnackBarAction(
+        label: "UNDO",
+        onPressed: () {
           config.onItemCreated(item);
-        }),
-      ]
+        }
+      )
     ));
   }
 

examples/material_gallery/lib/demo/snack_bar_demo.dart

@@ -32,16 +32,14 @@ class SnackBarDemo extends StatelessComponent {
             onPressed: () {
               Scaffold.of(context).showSnackBar(new SnackBar(
                 content: new Text('This is a SnackBar'),
-                actions: <SnackBarAction>[
-                  new SnackBarAction(
-                    label: 'Action',
-                    onPressed: () {
-                      Scaffold.of(context).showSnackBar(new SnackBar(
-                        content: new Text("You pressed the SnackBar's Action")
-                      ));
-                    }
-                  )
-                ]
+                action: new SnackBarAction(
+                  label: 'Action',
+                  onPressed: () {
+                    Scaffold.of(context).showSnackBar(new SnackBar(
+                      content: new Text("You pressed the SnackBar's Action")
+                    ));
+                  }
+                )
               ));
             }
           ),

examples/stocks/lib/stock_home.dart

@@ -210,9 +210,12 @@ class StockHomeState extends State<StockHome> {
     });
     _scaffoldKey.currentState.showSnackBar(new SnackBar(
       content: new Text("Purchased ${stock.symbol} for ${stock.lastSale}"),
-      actions: <SnackBarAction>[
-        new SnackBarAction(label: "BUY MORE", onPressed: () { _buyStock(stock, arrowKey); })
-      ]
+      action: new SnackBarAction(
+        label: "BUY MORE",
+        onPressed: () {
+          _buyStock(stock, arrowKey);
+        }
+      )
     ));
   }
 

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

@@ -74,7 +74,7 @@ class SnackBar extends StatelessComponent {
   SnackBar({
     Key key,
     this.content,
-    this.actions,
+    this.action,
     this.duration: kSnackBarShortDisplayDuration,
     this.animation
   }) : super(key: key) {
@@ -82,7 +82,7 @@ class SnackBar extends StatelessComponent {
   }
 
   final Widget content;
-  final List<SnackBarAction> actions;
+  final SnackBarAction action;
   final Duration duration;
   final Animation<double> animation;
 
@@ -99,8 +99,8 @@ class SnackBar extends StatelessComponent {
         )
       )
     ];
-    if (actions != null)
-      children.addAll(actions);
+    if (action != null)
+      children.add(action);
     CurvedAnimation heightAnimation = new CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
     CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve);
     ThemeData theme = Theme.of(context);
@@ -156,7 +156,7 @@ class SnackBar extends StatelessComponent {
     return new SnackBar(
       key: key ?? fallbackKey,
       content: content,
-      actions: actions,
+      action: action,
       duration: duration,
       animation: newAnimation
     );