Add support for converting elements to plugin API objects

pkg/analyzer_plugin/test/support/abstract_context.dart

Index: pkg/analyzer_plugin/test/support/abstract_context.dart
diff --git a/pkg/analyzer_plugin/test/support/abstract_context.dart b/pkg/analyzer_plugin/test/support/abstract_context.dart
index 2be9479e79ef5e19faa92fbf26be1e8ca822cf57..9ae61de9cb781faff6d333c89825618396baf364 100644
--- a/pkg/analyzer_plugin/test/support/abstract_context.dart
+++ b/pkg/analyzer_plugin/test/support/abstract_context.dart
@@ -79,7 +79,12 @@ class Required {
     driver.addFile(path);
     driver.changeFile(path);
     _fileContentOverlay[path] = content;
-    return null;
+    return provider.getFile(path).createSource();
+  }
+
+  Element findElementInUnit(CompilationUnit unit, String name,
+      [ElementKind kind]) {
+    return findChildElement(unit.element, name, kind);
   }
 
   File newFile(String path, [String content]) =>