Base JavaScript code position computation on JavaScript tracer.

pkg/compiler/lib/src/io/start_end_information.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.
 
 /// Source information system that maps spans of Dart AST nodes to spans of
 /// JavaScript nodes.
 
 library dart2js.source_information.start_end;
 
 import '../common.dart';
@@ skipping 98 matching lines @@
   String get shortText {
     return _computeText(startPosition.sourceUri.pathSegments.last);
   }
 
   String toString() {
     return _computeText('${startPosition.sourceUri}');
   }
 }
 
 class StartEndSourceInformationStrategy
-    implements JavaScriptSourceInformationStrategy {
+    extends JavaScriptSourceInformationStrategy {
   const StartEndSourceInformationStrategy();
 
   @override
   SourceInformationBuilder createBuilderForContext(AstElement element) {
     return new StartEndSourceInformationBuilder(element);
   }
 
   @override
   SourceInformationProcessor createProcessor(SourceMapper sourceMapper) {
     return new StartEndSourceInformationProcessor(sourceMapper);
@@ skipping 95 matching lines @@
 
   @override
   SourceInformationBuilder forContext(
       AstElement element, {SourceInformation sourceInformation}) {
     return new StartEndSourceInformationBuilder(element);
   }
 }