Migrate test block 7 to Dart 2.0.

tests/corelib_strong/duration_double_multiplication_test.dart

-// Copyright (c) 2013, 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";
-
-main() {
-  Duration d, d1;
-
-  d1 = new Duration(milliseconds: 1);
-  d = d1 * 0.005;
-  Expect.equals(1000 * 0.005, d.inMicroseconds);
-  d = d1 * 0.0;
-  Expect.equals(0, d.inMicroseconds);
-  d = d1 * -0.005;
-  Expect.equals(1000 * -0.005, d.inMicroseconds);
-  d = d1 * 0.0015;
-  Expect.equals((1000 * 0.0015).round(), d.inMicroseconds);
-}