dart2js: add --preserve-uris flag.

sdk/lib/_internal/compiler/implementation/js_emitter/old_emitter/emitter.dart

 // Copyright (c) 2014, 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.
 
 part of dart2js.js_emitter;
 
 
 class OldEmitter implements Emitter {
   final Compiler compiler;
   final CodeEmitterTask task;
@@ skipping 1191 matching lines @@
         #;
         return properties;
       }''', [debugCode]);
 
     mainBuffer.add(jsAst.prettyPrint(convertToFastObject, compiler));
     mainBuffer.add(N);
   }
 
   void writeLibraryDescriptors(CodeBuffer buffer, LibraryElement library) {
     var uri = "";
-    if (!compiler.enableMinification || backend.mustRetainUris) {
+    if (!compiler.enableMinification || backend.mustPreserveUris) {
       uri = library.canonicalUri;
       if (uri.scheme == 'file' && compiler.outputUri != null) {
         uri = relativize(compiler.outputUri, library.canonicalUri, false);
       }
     }
     ClassBuilder descriptor = elementDescriptors[library];
     if (descriptor == null) {
       // Nothing of the library was emitted.
       // TODO(floitsch): this should not happen. We currently have an example
       // with language/prefix6_negative_test.dart where we have an instance
@@ skipping 764 matching lines @@
     for (Element element in compiler.enqueuer.codegen.newlyEnqueuedElements) {
       if (element.isInstanceMember) {
         cachedClassBuilders.remove(element.enclosingClass);
 
         nativeEmitter.cachedBuilders.remove(element.enclosingClass);
 
       }
     }
   }
 }