Provide proper Dartium support for _getNewIsolateId().

lib/dom/templates/html/dartium/html_dartium.darttemplate

Index: lib/dom/templates/html/dartium/html_dartium.darttemplate
diff --git a/lib/dom/templates/html/dartium/html_dartium.darttemplate b/lib/dom/templates/html/dartium/html_dartium.darttemplate
index f7b9120b14a5686fa67da7989c50f260391276f3..0fea5e80fd330aa99806203249ab2301a060493e 100644
--- a/lib/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/lib/dom/templates/html/dartium/html_dartium.darttemplate
@@ -67,10 +67,7 @@ class _Null {
 
 final _null = const _Null();
 
-int _getNewIsolateId() {
-  // TODO(vsm): We need a Dartium native for this.
-  return 1;
-}
+int _getNewIsolateId() => _Utils._getNewIsolateId();
 
 bool _callPortInitialized = false;
 var _callPortLastResult = null;