Exposing DOM float & double types as double rather than num.

tests/html/audiobuffersourcenode_test.dart

 library AudioBufferSourceNodeTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/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();
-        node.start(ctx.currentTime, 0, 2);
+        node.start(ctx.currentTime, 0.0, 2.0);
         node.stop(ctx.currentTime + 2);
         expect(node is AudioBufferSourceNode, isTrue);
       }
     });
   });
 }