spawnDomUri stub for dart2js

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

Index: tools/dom/templates/html/dart2js/html_dart2js.darttemplate
diff --git a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
index 1d7918deed2648300331233c17176e13221725d6..91d162f96fa289851713df14a85b8bd3c1493d4a 100644
--- a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
+++ b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
@@ -132,3 +132,15 @@ class HtmlElement extends Element native "HTMLElement" {
 // older Chrome versions.
 // Issue #12573.
 abstract class _EntryArray implements List<Entry> native "EntryArray" {}
+
+/**
+ * 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): Implement this.
+  throw new UnimplementedError();
+}