Fixes for AnalysisDriver async CL comments.

pkg/analyzer/lib/src/dart/analysis/driver.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:typed_data';
 
 import 'package:analyzer/context/context_root.dart';
 import 'package:analyzer/context/declared_variables.dart';
@@ skipping 1295 matching lines @@
       var state = exception as _ExceptionState;
       exception = state.exception;
       stackTrace = state.stackTrace;
       contextKey = state.contextKey;
     }
     CaughtException caught = new CaughtException(exception, stackTrace);
     _exceptionController.add(new ExceptionResult(path, caught, contextKey));
   }
 
   /**
-   * Tests need a reliable way to simulate file changes during analysis.
+   * Runs any asynchronous work that was injected as part of a test using
+   * [AnalysisDriverTestView.workToWaitAfterComputingResult].
    *
-   * When a change happens, the driver must make sure that [getResult] produces
-   * results that include these changes. It is OK for the [results] stream
-   * to produce stale results as long as it eventually produces results that
-   * also include the changes.
+   * If the test view indicates that there is work to do, performs the work
+   * and returns a [Future] that will be signaled when the work completes.
+   *
+   * This gives tests a reliable way to simulate file changes during analysis.
    */
   Future _runTestAsyncWorkDuringAnalysis(String path) {
     var work = _testView.workToWaitAfterComputingResult;
     _testView.workToWaitAfterComputingResult = null;
-    return work != null ? work(path) : null;
+    return work != null ? work(path) : new Future.value();
   }
 
   /**
    * Serialize the given [resolvedUnit] errors and index into bytes.
    */
   List<int> _serializeResolvedUnit(
       CompilationUnit resolvedUnit, List<AnalysisError> errors) {
     AnalysisDriverUnitIndexBuilder index = indexUnit(resolvedUnit);
     return new AnalysisDriverResolvedUnitBuilder(
             errors: errors
@@ skipping 331 matching lines @@
 }
 
 @visibleForTesting
 class AnalysisDriverTestView {
   final AnalysisDriver driver;
 
   int numOfCreatedLibraryContexts = 0;
 
   int numOfAnalyzedLibraries = 0;
 
+  /**
+   * If non-null, a function that should be executed asynchronously after
+   * the next result is computed.
+   *
+   * This can be used by a test to simulate file changes during analysis.
+   */
   Future<Null> Function(String path) workToWaitAfterComputingResult;
 
   AnalysisDriverTestView(this.driver);
 
   FileTracker get fileTracker => driver._fileTracker;
 
   Map<String, AnalysisResult> get priorityResults => driver._priorityResults;
 
   Future<SummaryDataStore> getSummaryStore(String libraryPath) async {
     FileState library = driver.fsState.getFileForPath(libraryPath);
@@ skipping 419 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }