dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/js/js_source_mapping.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.source_mapping;
 
-import 'js.dart';
-import '../io/code_output.dart'
-    show BufferedCodeOutput, CodeBuffer, SourceLocations;
+import '../io/code_output.dart' show BufferedCodeOutput, SourceLocations;
 import '../io/source_information.dart'
     show SourceLocation, SourceInformation, SourceInformationStrategy;
+import 'js.dart';
 
 /// [SourceInformationStrategy] that can associate source information with
 /// JavaScript output.
 class JavaScriptSourceInformationStrategy extends SourceInformationStrategy {
   const JavaScriptSourceInformationStrategy();
 
   /// Creates a processor that can associate source information on [Node] with
   /// code offsets in the [sourceMapper].
   SourceInformationProcessor createProcessor(SourceMapper sourceMapper) {
     return const SourceInformationProcessor();
@@ skipping 32 matching lines @@
 
 /// A processor that associates [SourceInformation] with code position of
 /// JavaScript [Node]s.
 class SourceInformationProcessor extends CodePositionListener {
   const SourceInformationProcessor();
 
   /// Process the source information and code positions for the [node] and all
   /// its children.
   void process(Node node, BufferedCodeOutput code) {}
 }