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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 1307 matching lines @@
       // by name.
       enqueuer.registerGetOfStaticFunction(compiler.mainFunction);
     }
     if (enqueuer.isResolutionQueue) {
       for (String name in const [START_ROOT_ISOLATE,
                                  '_currentIsolate',
                                  '_callInIsolate']) {
         Element element = find(isolateHelperLibrary, name);
         enqueuer.addToWorkList(element);
         compiler.globalDependencies.registerDependency(element);
+        helpersUsed.add(element.declaration);
       }
     } else {
       enqueuer.addToWorkList(find(isolateHelperLibrary, START_ROOT_ISOLATE));
     }
   }
 
   bool isAssertMethod(Element element) => element == assertMethod;
 
   void registerRequiredType(DartType type, Element enclosingElement) {
     // If [argument] has type variables or is a type variable, this method
@@ skipping 1750 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }