Convert some tests to use the driver and prepare for others to be converted

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 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 80 matching lines @@
 ''');
 
   Source addPackageSource(String packageName, String filePath, String content) {
     packageMap[packageName] = [(newFolder('/pubcache/$packageName'))];
     File file = newFile('/pubcache/$packageName/$filePath', content);
     return file.createSource();
   }
 
   Source addSource(String path, String content, [Uri uri]) {
     File file = newFile(path, content);
+    Source source = file.createSource(uri);
     if (enableNewAnalysisDriver) {
       driver.addFile(path);
       driver.changeFile(path);
       _fileContentOverlay[path] = content;
-      return null;
     } else {
-      Source source = file.createSource(uri);
       ChangeSet changeSet = new ChangeSet();
       changeSet.addedSource(source);
       context.applyChanges(changeSet);
       context.setContents(source, content);
-      return source;
     }
+    return source;
   }
 
   File newFile(String path, [String content]) =>
       provider.newFile(provider.convertPath(path), content ?? '');
 
   Folder newFolder(String path) =>
       provider.newFolder(provider.convertPath(path));
 
   /**
    * Performs all analysis tasks in [context].
@@ skipping 92 matching lines @@
  * [engine.GeneralizingElementVisitor].
  */
 class _ElementVisitorFunctionWrapper extends GeneralizingElementVisitor {
   final _ElementVisitorFunction function;
   _ElementVisitorFunctionWrapper(this.function);
   visitElement(Element element) {
     function(element);
     super.visitElement(element);
   }
 }