Add MediaSource.supported, annotations for supported browsers, and a minimal test

tests/html/mediasource_test.dart

+library mediasource_test;
+
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_individual_config.dart';
+import 'dart:html';
+import 'dart:typed_data';
+import 'dart:async';

[terry, 2015/03/10 12:43:51]

Is this needed?

+
+main() {
+  useHtmlIndividualConfiguration();
+
+  var isMediaSource = predicate((x) => x is MediaSource, 'is a MediaSource');
+
+  group('supported', () {
+    test('supported', () {
+      expect(MediaSource.supported, true);
+    });
+  });
+
+  // TODO(alanknight): Actually exercise this, right now the tests are trivial.
+  group('functional', () {
+    var source;
+    if (MediaSource.supported) {
+      source = new MediaSource();
+    }
+
+    test('constructorTest', () {
+      if (MediaSource.supported) {
+        expect(source, isNotNull);
+        expect(source, isMediaSource);
+      }
+    });
+
+    test('media types', () {
+      if (MediaSource.supported) {
+        expect(MediaSource.isTypeSupported('text/html'), false);
+        expect(MediaSource.isTypeSupported('video/webm;codecs="vp8,vorbis"'),
+            true);
+      }
+    });
+  });
+}