Let operator* in Duration accept doubles.

tests/corelib/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);
+
+}