dart2js: get:hashCode methods to avoid Object.hashCode

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 83 matching lines @@
     StringBuffer sb = new StringBuffer();
     _createStructuredText(sb);
     return sb.toString();
   }
 
   /// Writes the structure of the constant into [sb].
   void _createStructuredText(StringBuffer sb);
 
   int _computeHashCode();
 
-  int get hashCode {
-    if (_hashCode == null) {
-      _hashCode = _computeHashCode();
-    }
-    return _hashCode;
-  }
+  int get hashCode => _hashCode ??= _computeHashCode();
 
   bool _equals(ConstantExpression other);
 
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! ConstantExpression) return false;
     if (kind != other.kind) return false;
     if (hashCode != other.hashCode) return false;
     return _equals(other);
   }
@@ skipping 1766 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }