Restrict backend misuse

pkg/compiler/lib/src/compiler.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.
 
 library dart2js.compiler_base;
 
 import 'dart:async' show
     EventSink,
     Future;
 
@@ skipping 307 matching lines @@
   // Initialized when dart:mirrors is loaded.
   ClassElement mirrorsUsedClass;
 
   // Initialized after mirrorSystemClass has been resolved.
   FunctionElement mirrorSystemGetNameFunction;
 
   // Initialized when dart:_internal is loaded.
   ClassElement symbolImplementationClass;
 
   // Initialized when symbolImplementationClass has been resolved.
+  // TODO(johnniwinther): Move this to [BackendHelpers].
   FunctionElement symbolValidatedConstructor;
 
   // Initialized when mirrorsUsedClass has been resolved.
   FunctionElement mirrorsUsedConstructor;
 
   // Initialized when dart:mirrors is loaded.
   ClassElement deferredLibraryClass;
 
   /// Document class from dart:mirrors.
   ClassElement documentClass;
@@ skipping 1753 matching lines @@
     if (_otherDependencies == null) {
       _otherDependencies = new Setlet<Element>();
     }
     _otherDependencies.add(element.implementation);
   }
 
   Iterable<Element> get otherDependencies {
     return _otherDependencies != null ? _otherDependencies : const <Element>[];
   }
 }