Native entry point for spawnDomUri

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

Index: tools/dom/templates/html/dartium/html_dartium.darttemplate
diff --git a/tools/dom/templates/html/dartium/html_dartium.darttemplate b/tools/dom/templates/html/dartium/html_dartium.darttemplate
index a8168c1f98e6d36da745a8acc5170ece8d7f807c..8c22b56209968955c8c7e481bf3436ae8cac8975 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -119,3 +119,15 @@ HtmlDocument get document {
   _document = window.document;
   return _document;
 }
+
+/**
+ * Spawn a DOM isolate using the given URI in the same window.
+ * This isolate is not concurrent.  It runs on the browser thread
+ * with full access to the DOM.
+ * Note: this API is still evolving and may move to dart:isolate.
+ */
+@Experimental()
+Future<Isolate> spawnDomUri(Uri uri, List<String> args, message) {
+  // TODO(17738): Plumb arguments and return value through.
+  return _Utils.spawnDomUri(uri.toString());
+}