Support an @OnPlatform annotation.

test/backend/metadata_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. 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:test/src/backend/metadata.dart';
 import 'package:test/src/backend/test_platform.dart';
 import 'package:test/src/frontend/timeout.dart';
 import 'package:test/src/frontend/skip.dart';
 import 'package:test/test.dart';
 
 void main() {
   group("onPlatform", () {
     test("parses a valid map", () {
       var metadata = new Metadata.parse(onPlatform: {
         "chrome": new Timeout.factor(2),
         "vm": [new Skip(), new Timeout.factor(3)]
       });
 
       var key = metadata.onPlatform.keys.first;
       expect(key.evaluate(TestPlatform.chrome), isTrue);
       expect(key.evaluate(TestPlatform.vm), isFalse);
       var value = metadata.onPlatform.values.first;
       expect(value.timeout.scaleFactor, equals(2));
 
-
       key = metadata.onPlatform.keys.last;
       expect(key.evaluate(TestPlatform.vm), isTrue);
       expect(key.evaluate(TestPlatform.chrome), isFalse);
       value = metadata.onPlatform.values.last;
       expect(value.skip, isTrue);
       expect(value.timeout.scaleFactor, equals(3));
     });
 
     test("refuses an invalid value", () {
       expect(() {
@@ skipping 21 matching lines @@
       }, throwsArgumentError);
     });
 
     test("refuses multiple Skips", () {
       expect(() {
         new Metadata.parse(onPlatform: {"chrome": [new Skip(), new Skip()]});
       }, throwsArgumentError);
     });
   });
 }