Use file/offset/kind triplet as elements id.

pkg/analysis_server/test/abstract_context.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.
 
 library testing.abstract_context;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
@@ skipping 26 matching lines @@
 typedef void _ElementVisitorFunction(Element element);
 
 class AbstractContextTest {
   static final DartSdk SDK = new MockSdk();
   static final UriResolver SDK_RESOLVER = new DartUriResolver(SDK);
 
   MemoryResourceProvider provider = new MemoryResourceProvider();
   UriResolver resourceResolver;
   AnalysisContext context;
 
-  Source addSource(String path, String content) {
+  Source addSource(String path, String content, [Uri uri]) {
     File file = provider.newFile(path, content);
-    Source source = file.createSource();
+    Source source = file.createSource(uri);
     ChangeSet changeSet = new ChangeSet();
     changeSet.addedSource(source);
     context.applyChanges(changeSet);
     context.setContents(source, content);
     return source;
   }
 
   /**
    * Performs all analysis tasks in [context].
    */
@@ skipping 31 matching lines @@
  * [engine.GeneralizingElementVisitor].
  */
 class _ElementVisitorFunctionWrapper extends GeneralizingElementVisitor {
   final _ElementVisitorFunction function;
   _ElementVisitorFunctionWrapper(this.function);
   visitElement(Element element) {
     function(element);
     super.visitElement(element);
   }
 }