Add a getter to TypeProvider for Future<Null>.

pkg/analyzer/lib/src/generated/error_verifier.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 engine.resolver.error_verifier;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'ast.dart';
@@ skipping 5760 matching lines @@
       }
     }
     // done
     return hasProblem;
   }
 
   DartType _computeReturnTypeForMethod(Expression returnExpression) {
     // TODO(paulberry): do the right thing for generators.
     if (returnExpression == null) {
       if (_enclosingFunction.isAsynchronous) {
-        return _typeProvider.futureType.substitute4(
-            <DartType>[_typeProvider.nullType]);
+        return _typeProvider.futureNullType;
       } else {
         return VoidTypeImpl.instance;
       }
     }
     DartType staticReturnType = getStaticType(returnExpression);
     if (staticReturnType != null &&
         _enclosingFunction.isAsynchronous &&
         staticReturnType.element != _typeProvider.futureType.element) {
       return _typeProvider.futureType.substitute4(<DartType>[staticReturnType]);
     }
@@ skipping 436 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }