Use AbstractSingleUnitTest, so MockSdk in resynthesize tests.

pkg/analyzer/test/src/abstract_single_unit.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 test.services.src.index.abstract_single_file;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/ast/utilities.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:unittest/unittest.dart';
 
 import 'context/abstract_context.dart';
 
 class AbstractSingleUnitTest extends AbstractContextTest {
   bool verifyNoTestUnitErrors = true;
 
   String testCode;
   String testFile = '/test.dart';
   Source testSource;
   CompilationUnit testUnit;
   CompilationUnitElement testUnitElement;
   LibraryElement testLibraryElement;
 
-  void addTestSource(String code, [Uri uri]) {
+  Source addTestSource(String code, [Uri uri]) {
     testCode = code;
     testSource = addSource(testFile, code);
+    return testSource;
   }
 
   void assertNoErrorsInSource(Source source) {
     List<AnalysisError> errors = context.getErrors(source).errors;
     expect(errors, isEmpty);
   }
 
   Element findElement(String name, [ElementKind kind]) {
     return findChildElement(testUnitElement, name, kind);
   }
@@ skipping 61 matching lines @@
   void resolveTestUnit(String code) {
     addTestSource(code);
     testUnit = resolveLibraryUnit(testSource);
     if (verifyNoTestUnitErrors) {
       assertNoErrorsInSource(testSource);
     }
     testUnitElement = testUnit.element;
     testLibraryElement = testUnitElement.library;
   }
 }