Complete with null, not return null as a Future.

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/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 487 matching lines @@
     return task.completer.future;
   }
 
   /**
    * Return a [Future] that completes with the [AnalysisDriverUnitIndex] for
    * the file with the given [path], or with `null` if the file cannot be
    * analyzed.
    */
   Future<AnalysisDriverUnitIndex> getIndex(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
     var completer = new Completer<AnalysisDriverUnitIndex>();
     _indexRequestedFiles
         .putIfAbsent(path, () => <Completer<AnalysisDriverUnitIndex>>[])
         .add(completer);
     _statusSupport.transitionToAnalyzing();
     _scheduler._notify(this);
     return completer.future;
   }
 
   /**
    * Return a [Future] that completes with a [AnalysisResult] for the Dart
    * file with the given [path]. If the file is not a Dart file or cannot
    * be analyzed, the [Future] completes with `null`.
    *
    * The [path] must be absolute and normalized.
    *
    * The [path] can be any file - explicitly or implicitly analyzed, or neither.
    *
    * If the driver has the cached analysis result for the file, it is returned.
    *
    * Otherwise causes the analysis state to transition to "analyzing" (if it is
    * not in that state already), the driver will read the file and produce the
    * analysis result for it, which is consistent with the current file state
    * (including the new state of the file), prior to the next time the analysis
    * state transitions to "idle".
    */
   Future<AnalysisResult> getResult(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
 
     // Return the cached result.
     {
       AnalysisResult result = _priorityResults[path];
       if (result != null) {
         return new Future.value(result);
       }
     }
 
@@ skipping 19 matching lines @@
     _scheduler._notify(this);
     return task.completer.future;
   }
 
   /**
    * Return a [Future] that completes with the [CompilationUnitElement] for the
    * file with the given [path], or with `null` if the file cannot be analyzed.
    */
   Future<CompilationUnitElement> getUnitElement(String path) {
     if (!_fsState.hasUri(path)) {
-      return null;
+      return new Future.value();
     }
     var completer = new Completer<CompilationUnitElement>();
     _unitElementRequestedFiles
         .putIfAbsent(path, () => <Completer<CompilationUnitElement>>[])
         .add(completer);
     _statusSupport.transitionToAnalyzing();
     _scheduler._notify(this);
     return completer.future;
   }
 
@@ skipping 1106 matching lines @@
           libraryDeclarations.add(new TopLevelDeclarationInSource(
               file.source, declaration, isExported));
         }
       }
     }
 
     // We're not done yet.
     return false;
   }
 }