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

pkg/analyzer/test/src/task/strong/checker_test.dart

Index: pkg/analyzer/test/src/task/strong/checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/checker_test.dart b/pkg/analyzer/test/src/task/strong/checker_test.dart
index 9401f1a6a52ed166afadc16ec48626094703c90a..a788da18a9d26cc41553afda8b9482b744e77b99 100644
--- a/pkg/analyzer/test/src/task/strong/checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/checker_test.dart
@@ -937,16 +937,16 @@ 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;