Migrate test block 9 to Dart 2.0

tests/corelib_2/hashcode_boxed_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";
 
 double fib(double n) {
   return n <= 1.0 ? 1.0 : fib(n - 1) + fib(n - 2);
 }
 
 main() {
   // Compute the same value in a way that won't be optimized away so the results
   // are different objects in memory.
   var a = fib(5.0) + 1.0;
   var b = fib(4.0) + 4.0;
 
   Expect.isTrue(identical(a, b));
   Expect.equals(identityHashCode(a), identityHashCode(b));
   Expect.equals(a, b);
   Expect.equals(a.hashCode, b.hashCode);
 }