Make hashCode a getter and not a method.

tests/standalone/medium_integer_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.
 // Testing Mints. Note that the tests may not work on 64-bit machines,
 // as Smi's would be used to represent many of the numbers.
 
 #library("MediumIntegerTest.dart");
 
 
 class MediumIntegerTest {
@@ skipping 83 matching lines @@
     testSmiOverflow();
     testMintAdd();
     testMintSub();
     testMintMul();
     testMintDiv();
     testMintAnd(-1925149952);
     testMintAnd(1925149952);
     testMintAnd(0x100000001);
     var a = 100000000000;
     var b = 100000000001;
-    Expect.equals(false, a.hashCode() == b.hashCode());
-    Expect.equals(true, a.hashCode() == (b - 1).hashCode());
+    Expect.equals(false, a.hashCode == b.hashCode);
+    Expect.equals(true, a.hashCode == (b - 1).hashCode);
   }
 }
 
 main() {
   for (int i = 0; i < 4000; i++) {
     MediumIntegerTest.testMain();
   }
 }