SnackBarActions shouldn't be tappable twice

They should automagically disable after the first tap.

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

@@ -36,11 +36,13 @@ const Curve _snackBarFadeCurve = const Interval(0.72, 1.0, curve: Curves.fastOut
 /// Snack bar actions are always enabled. If you want to disable a snack bar
 /// action, simply don't include it in the snack bar.
 ///
+/// Snack bar actions can only be pressed once. Subsequent presses are ignored.
+///
 /// See also:
 ///
 ///  * [SnackBar]
 ///  * <https://www.google.com/design/spec/components/snackbars-toasts.html>
-class SnackBarAction extends StatelessWidget {
+class SnackBarAction extends StatefulWidget {
   SnackBarAction({Key key, this.label, this.onPressed }) : super(key: key) {
     assert(label != null);
     assert(onPressed != null);
@@ -50,16 +52,35 @@ class SnackBarAction extends StatelessWidget {
   final String label;
 
   /// The callback to be invoked when the button is pressed. Must be non-null.
+  ///
+  /// This callback will be invoked at most once each time this action is
+  /// displayed in a [SnackBar].
   final VoidCallback onPressed;
 
+  @override
+  _SnackBarActionState createState() => new _SnackBarActionState();
+}
+
+class _SnackBarActionState extends State<SnackBarAction> {
+  bool _haveTriggeredAction = false;
+
+  void _handlePressed() {
+    if (_haveTriggeredAction)
+      return;
+    setState(() {
+      _haveTriggeredAction = true;
+    });
+    config.onPressed();
+  }
+
   @override
   Widget build(BuildContext context) {
     return new Container(
       margin: const EdgeInsets.only(left: _kSideMargins),
       child: new FlatButton(
-        onPressed: onPressed,
+        onPressed: _haveTriggeredAction ? null : _handlePressed,
         textTheme: ButtonColor.accent,
-        child: new Text(label)
+        child: new Text(config.label)
       )
     );
   }

packages/flutter/test/widget/snack_bar_test.dart → packages/flutter/test/material/snack_bar_test.dart

@@ -282,4 +282,48 @@ void main() {
     });
   });
 
+  test('SnackBar cannot be tapped twice', () {
+    testWidgets((WidgetTester tester) {
+      int tapCount = 0;
+      tester.pumpWidget(new MaterialApp(
+        routes: <String, WidgetBuilder>{
+          '/': (BuildContext context) {
+            return new Scaffold(
+              body: new Builder(
+                builder: (BuildContext context) {
+                  return new GestureDetector(
+                    onTap: () {
+                      Scaffold.of(context).showSnackBar(new SnackBar(
+                        content: new Text('I am a snack bar.'),
+                        duration: new Duration(seconds: 2),
+                        action: new SnackBarAction(
+                          label: 'ACTION',
+                          onPressed: () {
+                            ++tapCount;
+                          }
+                        )
+                      ));
+                    },
+                    child: new Text('X')
+                  );
+                }
+              )
+            );
+          }
+        }
+      ));
+      tester.tap(tester.findText('X'));
+      tester.pump(); // start animation
+      tester.pump(const Duration(milliseconds: 750));
+
+      expect(tapCount, equals(0));
+      tester.tap(tester.findText('ACTION'));
+      expect(tapCount, equals(1));
+      tester.tap(tester.findText('ACTION'));
+      expect(tapCount, equals(1));
+      tester.pump();
+      tester.tap(tester.findText('ACTION'));
+      expect(tapCount, equals(1));
+    });
+  });
 }