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

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 164fb39bd3f7da3a119f3e73205da4aefecfb4dc..7bf514e811d24da2740e185d7425badc65e3ccfd 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1325,6 +1325,7 @@ class JavaScriptBackend extends Backend {
         Element element = find(isolateHelperLibrary, name);
         enqueuer.addToWorkList(element);
         compiler.globalDependencies.registerDependency(element);
+        helpersUsed.add(element.declaration);
       }
     } else {
       enqueuer.addToWorkList(find(isolateHelperLibrary, START_ROOT_ISOLATE));