Migrate language/arithmetic2_test ... language/async_await_syntax_test.

tests/language_2/assign_instance_method_test.dart

 // Copyright (c) 2011, 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 'package:expect/expect.dart';
+
 class A {
   A() {}
   imethod() {
     return 0;
   }
 }
 
 main() {
-  var a = new A();
-  // Illegal, can't change a member method
-  a.imethod = () {
-    return 1;
-  };
+  dynamic a = new A();
+
+  // Illegal, can't change a member method.
+  Expect.throws(() {
+    a.imethod = () {
+      return 1;
+    };
+  });
 }