Make Future.value() constructor in MockSdk consistent with the actual SDK.

pkg/analyzer/test/src/task/strong/checker_test.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 analyzer.test.src.task.strong.checker_test;
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_test_helper.dart';
 
@@ skipping 919 matching lines @@
   void test_functionModifiers_async() {
     checkFile('''
 import 'dart:async';
 import 'dart:math' show Random;
 
 dynamic x;
 
 foo1() async => x;
 Future foo2() async => x;
 Future<int> foo3() async => x;
-Future<int> foo4() async => new Future<int>.value(/*info:DYNAMIC_CAST*/x);
+Future<int> foo4() async => new Future<int>.value(x);
 Future<int> foo5() async =>
-    /*error:RETURN_OF_INVALID_TYPE*/new Future<String>.value(/*info:DYNAMIC_CAST*/x);
+    /*error:RETURN_OF_INVALID_TYPE*/new Future<String>.value(x);
 
 bar1() async { return x; }
 Future bar2() async { return x; }
 Future<int> bar3() async { return x; }
-Future<int> bar4() async { return new Future<int>.value(/*info:DYNAMIC_CAST*/x); }
+Future<int> bar4() async { return new Future<int>.value(x); }
 Future<int> bar5() async {
-  return /*error:RETURN_OF_INVALID_TYPE*/new Future<String>.value(/*info:DYNAMIC_CAST*/x);
+  return /*error:RETURN_OF_INVALID_TYPE*/new Future<String>.value(x);
 }
 
 int y;
 Future<int> z;
 
 baz() async {
   int a = /*info:DYNAMIC_CAST*/await x;
   int b = await y;
   int c = await z;
   String d = /*error:INVALID_ASSIGNMENT*/await z;
@@ skipping 3067 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }