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

pkg/analysis_server/test/abstract_context.dart

Index: pkg/analysis_server/test/abstract_context.dart
diff --git a/pkg/analysis_server/test/abstract_context.dart b/pkg/analysis_server/test/abstract_context.dart
index 81464a946bce7e2db67ddc6ffed937730b742304..34986c12761702a51d586f664d5728dd375bcf6e 100644
--- a/pkg/analysis_server/test/abstract_context.dart
+++ b/pkg/analysis_server/test/abstract_context.dart
@@ -98,19 +98,18 @@ class Required {
 
   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]) =>