Use memory compiler in compiler_helper.dart.

pkg/compiler/lib/src/js_backend/namer_names.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.
 
 part of js_backend;
 
 abstract class _NamerName extends jsAst.Name {
   int get _kind;
   _NamerName get _target => this;
 
@@ skipping 175 matching lines @@
   bool operator==(other) {
     if (other is _NameReference) other = other._target;
     if (identical(this, other)) return true;
     return false;
   }
 
   @override
   int get hashCode => super.hashCode;
 
   finalize() {
-    assert(!isFinalized);
+    assert(invariant(NO_LOCATION_SPANNABLE, !isFinalized,
+        message: "TokenName($key)=$_name has already been finalized."));
     _name = _scope.getNextName();
   }
 }
 
 class _NameReference extends _NamerName implements jsAst.AstContainer {
   _NamerName _target;
 
   int get _kind => _target._kind;
   String get key => _target.key;
 
   Iterable<jsAst.Node> get containedNodes => [_target];
 
   _NameReference(this._target);
 
   String get name => _target.name;
 
   @override
   int compareTo(_NamerName other) => _target.compareTo(other);
 
   @override
   bool operator==(other) => _target == other;
 
   @override
   int get hashCode => _target.hashCode;
 }