Use common.dart to re-export commonly used entities.

pkg/compiler/lib/src/parser/parser_task.dart

 // Copyright (c) 2011, 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.parser.task;
 
+import '../common.dart';
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
-import '../diagnostics/invariant.dart' show
-    invariant;
 import '../elements/modelx.dart' show
     ElementX;
 import '../tokens/token.dart' show
     Token;
 import '../tree/tree.dart' show
     Node;
 
 import 'listener.dart' show
     ParserError;
 import 'node_listener.dart' show
@@ skipping 18 matching lines @@
       } on ParserError catch(_) {
         assert(invariant(token, compiler.compilationFailed));
         return listener.makeNodeList(0, null, null, '\n');
       }
       Node result = listener.popNode();
       assert(listener.nodes.isEmpty);
       return result;
     });
   }
 }