Make subclasses of AbstractContextTest asynchronous.

pkg/analysis_server/test/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 'dart:async';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/dart/ast/utilities.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:test/test.dart';
 
 import 'abstract_context.dart';
 
@@ skipping 74 matching lines @@
       }
       if (c >= '0'.codeUnitAt(0) && c <= '9'.codeUnitAt(0)) {
         length++;
         continue;
       }
       break;
     }
     return length;
   }
 
-  void resolveTestUnit(String code) {
+  Future<Null> resolveTestUnit(String code) async {
     addTestSource(code);
-    testUnit = resolveLibraryUnit(testSource);
+    testUnit = await resolveLibraryUnit(testSource);
     if (verifyNoTestUnitErrors) {
       assertNoErrorsInSource(testSource);
     }
     testUnitElement = testUnit.element;
     testLibraryElement = testUnitElement.library;
   }
 }