Make CompilerTask independent of compiler.

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

 // Copyright (c) 2015, 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.
 
 library dart2js.js_backend.helpers;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers, Uris;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
@@ skipping 535 matching lines @@
 
   Element get createRuntimeType {
     return findHelper('createRuntimeType');
   }
 
   Element get fallThroughError {
     return findHelper("getFallThroughError");
   }
 
   Element get createInvocationMirror {
-    return findHelper(Compiler.CREATE_INVOCATION_MIRROR);
+    return findHelper('createInvocationMirror');
   }
 
   Element get cyclicThrowHelper {
     return findHelper("throwCyclicInit");
   }
 
   Element get asyncHelper {
     return findAsyncHelper("_asyncHelper");
   }
 
@@ skipping 139 matching lines @@
   MethodElement _objectNoSuchMethod;
 
   MethodElement get objectNoSuchMethod {
     if (_objectNoSuchMethod == null) {
       _objectNoSuchMethod =
           coreClasses.objectClass.lookupLocalMember(Identifiers.noSuchMethod_);
     }
     return _objectNoSuchMethod;
   }
 }