Clean up and improve some of the task tests

pkg/analyzer/test/src/context/abstract_context.dart

 // Copyright (c) 2015, 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.src.task.abstract_context_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
@@ skipping 54 matching lines @@
         }
         fail(buffer.toString());
       }
     }
     expect(elements, hasLength(names.length));
   }
 
   /**
    * Compute the given [result] for the given [target].
    */
-  void computeResult(AnalysisTarget target, ResultDescriptor result) {
+  void computeResult(AnalysisTarget target, ResultDescriptor result,
+      {isInstanceOf matcher: null}) {
     oldOutputs = outputs;
     task = analysisDriver.computeResult(target, result);
-    expect(task, isNotNull);
+    if (matcher == null) {
+      expect(task, isNotNull);
+    } else {
+      expect(task, matcher);
+    }
     expect(task.caughtException, isNull);
     outputs = task.outputs;
+    for (ResultDescriptor descriptor in task.descriptor.results) {
+      expect(outputs, contains(descriptor));
+    }
   }
 
   AnalysisContextImpl createAnalysisContext() {
     return new AnalysisContextImpl();
   }
 
   Source newSource(String path, [String content = '']) {
     File file = resourceProvider.newFile(path, content);
     return file.createSource();
   }
@@ skipping 20 matching lines @@
     analysisCache = context.analysisCache;
     analysisDriver = context.driver;
   }
 
   void setUp() {
     prepareAnalysisContext();
   }
 
   void tearDown() {}
 }