Implement hashCode on objects stored in a set or used as map keys.

dart/sdk/lib/_internal/compiler/implementation/constants.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.
 
 part of dart2js;
 
 abstract class ConstantVisitor<R> {
   R visitSentinel(SentinelConstant constant);
   R visitFunction(FunctionConstant constant);
   R visitNull(NullConstant constant);
@@ skipping 90 matching lines @@
   const PrimitiveConstant();
   bool isPrimitive() => true;
 
   bool operator ==(var other) {
     if (other is !PrimitiveConstant) return false;
     PrimitiveConstant otherPrimitive = other;
     // We use == instead of 'identical' so that DartStrings compare correctly.
     return value == otherPrimitive.value;
   }
 
+  int get hashCode => throw new UnsupportedError('PrimitiveConstant.hashCode');
+
   String toString() => value.toString();
   // Primitive constants don't have dependencies.
   List<Constant> getDependencies() => const <Constant>[];
   DartString toDartString();
 }
 
 class NullConstant extends PrimitiveConstant {
   /** The value a Dart null is compiled to in JavaScript. */
   static const String JsNull = "null";
 
@@ skipping 373 matching lines @@
     for (int i = 0; i < fields.length; i++) {
       if (fields[i] != other.fields[i]) return false;
     }
     return true;
   }
 
   List<Constant> getDependencies() => fields;
 
   accept(ConstantVisitor visitor) => visitor.visitConstructed(this);
 }