Sped up hashCode by removing megamorphic call to _identityHashCode.

runtime/lib/integers.dart

 // Copyright (c) 2012, 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.
 
 abstract class _IntegerImplementation {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
 
   Type get runtimeType => int;
 
@@ skipping 390 matching lines @@
     }
     return _binaryGcd(x, y, false);
   }
 }
 
 class _Smi extends _IntegerImplementation implements int {
   factory _Smi._uninstantiable() {
     throw new UnsupportedError(
         "_Smi can only be allocated by the VM");
   }
+  int get hashCode => this;
   int get _identityHashCode => this;
-  int get hashCode => this;
   int operator ~() native "Smi_bitNegate";
   int get bitLength native "Smi_bitLength";
 
   int operator &(int other) => other._bitAndFromSmi(this);
 
   int _bitAndFromSmi(int other) native "Smi_bitAndFromSmi";
   int _shrFromInt(int other) native "Smi_shrFromInt";
   int _shlFromInt(int other) native "Smi_shlFromInt";
 
   /**
@@ skipping 178 matching lines @@
     return result;
   }
 }
 
 // Represents integers that cannot be represented by Smi but fit into 64bits.
 class _Mint extends _IntegerImplementation implements int {
   factory _Mint._uninstantiable() {
     throw new UnsupportedError(
         "_Mint can only be allocated by the VM");
   }
+  int get hashCode => this;
   int get _identityHashCode => this;
-  int get hashCode => this;
   int operator ~() native "Mint_bitNegate";
   int get bitLength native "Mint_bitLength";
 
   int _bitAndFromSmi(int other) => _bitAndFromInteger(other);
 
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }