If the expected 'returnType' is 'null', just do not check it.

pkg/analysis_server/test/services/completion/dart/completion_contributor_util.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.completion.dart.util;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart' as protocol
     show Element, ElementKind;
@@ skipping 244 matching lines @@
       String importUri}) {
     CompletionSuggestion cs = assertSuggest(name,
         csKind: kind,
         relevance: relevance,
         importUri: importUri,
         isDeprecated: isDeprecated);
     if (returnType != null) {
       expect(cs.returnType, returnType);
     } else if (isNullExpectedReturnTypeConsideredDynamic) {
       expect(cs.returnType, 'dynamic');
-    } else {
-      expect(cs.returnType, isNull);
     }
     protocol.Element element = cs.element;
     expect(element, isNotNull);
     expect(element.kind, equals(protocol.ElementKind.FUNCTION));
     expect(element.name, equals(name));
     expect(element.isDeprecated, equals(isDeprecated));
     String param = element.parameters;
     expect(param, isNotNull);
     expect(param[0], equals('('));
     expect(param[param.length - 1], equals(')'));
     if (returnType != null) {
       expect(element.returnType, returnType);
     } else if (isNullExpectedReturnTypeConsideredDynamic) {
       expect(element.returnType, 'dynamic');
-    } else {
-      expect(element.returnType, isNull);
     }
     assertHasParameterInfo(cs);
     return cs;
   }
 
   CompletionSuggestion assertSuggestFunctionTypeAlias(
       String name, String returnType,
       {bool isDeprecated: false,
       int relevance: DART_RELEVANCE_DEFAULT,
       CompletionSuggestionKind kind: CompletionSuggestionKind.INVOCATION,
@@ skipping 119 matching lines @@
   CompletionSuggestion assertSuggestTopLevelVar(String name, String returnType,
       {int relevance: DART_RELEVANCE_DEFAULT,
       CompletionSuggestionKind kind: CompletionSuggestionKind.INVOCATION,
       String importUri}) {
     CompletionSuggestion cs = assertSuggest(name,
         csKind: kind, relevance: relevance, importUri: importUri);
     if (returnType != null) {
       expect(cs.returnType, returnType);
     } else if (isNullExpectedReturnTypeConsideredDynamic) {
       expect(cs.returnType, 'dynamic');
-    } else {
-      expect(cs.returnType, isNull);
     }
     protocol.Element element = cs.element;
     expect(element, isNotNull);
     expect(element.kind, equals(protocol.ElementKind.TOP_LEVEL_VARIABLE));
     expect(element.name, equals(name));
     expect(element.parameters, isNull);
     if (returnType != null) {
       expect(element.returnType, returnType);
     } else if (isNullExpectedReturnTypeConsideredDynamic) {
       expect(element.returnType, 'dynamic');
-    } else {
-      expect(element.returnType, isNull);
     }
     assertHasNoParameterInfo(cs);
     return cs;
   }
 
   /**
    * Return a [Future] that completes with the containing library information
    * after it is accessible via [context.getLibrariesContaining].
    */
   Future computeLibrariesContaining([int times = 200]) {
@@ skipping 121 matching lines @@
   }
 
   @override
   void setUp() {
     super.setUp();
     index = createMemoryIndex();
     searchEngine = new SearchEngineImpl(index);
     contributor = createContributor();
   }
 }