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

pkg/compiler/lib/src/js/js.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 js;
 
 import 'package:js_ast/js_ast.dart';
 export 'package:js_ast/js_ast.dart';
 
+import '../common.dart';
 import '../compiler.dart' show
     Compiler;
-import '../diagnostics/diagnostic_listener.dart' show
-    DiagnosticReporter;
-import '../diagnostics/spannable.dart' show
-    NO_LOCATION_SPANNABLE;
 import '../dump_info.dart' show
     DumpInfoTask;
 import '../io/code_output.dart' show
     CodeBuffer,
     CodeOutput;
 import '../js_emitter/js_emitter.dart' show
     USE_LAZY_EMITTER;
 
 import 'js_source_mapping.dart';
 
@@ skipping 133 matching lines @@
         }
       }
       _cachedLiteral = js.escapedString(text);
     }
     return _cachedLiteral;
   }
 
   @override
   String get value => _literal.value;
 }