Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/audiobuffersourcenode_test.dart

+library AudioBufferSourceNodeTest;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_individual_config.dart';
+import 'dart:web_audio';
+
+main() {
+
+  useHtmlIndividualConfiguration();
+
+  group('supported', () {
+    test('supported', () {
+      expect(AudioContext.supported, true);
+    });
+  });
+
+  group('functional', () {
+    test('createBuffer', () {
+      if(AudioContext.supported) {
+        var ctx = new AudioContext();
+        AudioBufferSourceNode node = ctx.createBufferSource();
+        expect(node is AudioBufferSourceNode, isTrue);
+        node.start(ctx.currentTime, 0, 2);
+        expect(node is AudioBufferSourceNode, isTrue);
+      }
+    });
+  });
+}