dart2js: Mark startRootIsolate as "used" by the backend.

sdk/lib/_internal/js_runtime/lib/isolate_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
index b8a44b4d72a7670385f4af0e72d38dee8717296f..a6edabbf03efa26660f7e31d96beb1d5cad06ca3 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
@@ -96,9 +96,6 @@ _IsolateContext _currentIsolate() => _globalState.currentContext;
  * call is not emitted.
  */
 void startRootIsolate(entry, args) {
-  // The dartMainRunner can inject a new arguments array. We pass the arguments
-  // through a "JS", so that the type-inferrer loses track of it.
-  args = JS("", "#", args);
   if (args == null) args = [];
   if (args is! List) {
     throw new ArgumentError("Arguments to main must be a List: $args");