Provide a fallback for AudioBufferSourceNode.start to noteOn and stop to

tests/html/audiobuffersourcenode_test.dart

+#library('AudioBufferSourceNodeTest');
+#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/html_config.dart');
+#import('dart:html');
+
+main() {
+
+  useHtmlConfiguration();
+
+  test('createBuffer', () {
+      var ctx = new AudioContext();
+      AudioBufferSourceNode node = ctx.createBufferSource();
+      node.start(ctx.currentTime, 0, 2);
+      node.stop(ctx.currentTime + 2);
+      expect(node is AudioBufferSourceNode);
+  });
+}