dart2js cleanup: remove unused imports and variables

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 '../dump_info.dart' show DumpInfoTask;
-import '../io/code_output.dart' show CodeBuffer, CodeOutput;
+import '../io/code_output.dart' show CodeBuffer;
 import '../js_emitter/js_emitter.dart' show USE_LAZY_EMITTER;
+import 'js_source_mapping.dart';
 
-import 'js_source_mapping.dart';
+export 'package:js_ast/js_ast.dart';
 
 String prettyPrint(Node node, Compiler compiler,
     {bool allowVariableMinification: true,
     Renamer renamerForNames: JavaScriptPrintingOptions.identityRenamer}) {
   // TODO(johnniwinther): Do we need all the options here?
   JavaScriptPrintingOptions options = new JavaScriptPrintingOptions(
       shouldCompressOutput: compiler.options.enableMinification,
       minifyLocalVariables: allowVariableMinification,
       preferSemicolonToNewlineInMinifiedOutput: USE_LAZY_EMITTER,
       renamerForNames: renamerForNames);
@@ skipping 160 matching lines @@
   }
   if (node is PropertyAccess) {
     PropertyAccess access = node;
     if (access.receiver is InterpolatedExpression) {
       InterpolatedExpression hole = access.receiver;
       return hole.isPositional && hole.nameOrPosition == 0;
     }
   }
   return false;
 }