Very simple version of Isolates.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 240fb2dc02d8daa88696da8f59a134e7efaafa93..2c4b4fe2bf738ef3f39ac153753b7d2a25a08826 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -25599,13 +25599,13 @@ class Url extends NativeFieldWrapperClass2 {
     if ((blob_OR_source_OR_stream is Blob || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_1(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is MediaSource || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is MediaStream || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_2(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is _WebKitMediaSource || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is MediaSource || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_3(blob_OR_source_OR_stream);
     }
-    if ((blob_OR_source_OR_stream is MediaStream || blob_OR_source_OR_stream == null)) {
+    if ((blob_OR_source_OR_stream is _WebKitMediaSource || blob_OR_source_OR_stream == null)) {
       return _createObjectURL_4(blob_OR_source_OR_stream);
     }
     throw new ArgumentError("Incorrect number or type of arguments");
@@ -31209,6 +31209,8 @@ class _JsDeserializer extends _Deserializer {
   }
 }
 
+class SendPortSync {}
+
 // The receiver is JS.
 class _JsSendPortSync implements SendPortSync {