Warn about overriding operator== but not hashCode.

dart/tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 78 matching lines @@
 
 const String FOREIGN_LIBRARY = r'''
   dynamic JS(String typeDescription, String codeTemplate,
     [var arg0, var arg1, var arg2, var arg3, var arg4, var arg5, var arg6,
      var arg7, var arg8, var arg9, var arg10, var arg11]) {}''';
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
   class Interceptor {
     toString() {}
     bool operator==(other) => identical(this, other);
+    get hashCode => throw "Interceptor.hashCode not implemented.";
     noSuchMethod(im) { throw im; }
   }
   abstract class JSIndexable {
     get length;
   }
   abstract class JSMutableIndexable extends JSIndexable {}
   class JSArray extends Interceptor implements List, JSIndexable {
     var length;
     operator[](index) => this[index];
     operator[]=(index, value) {}
@@ skipping 21 matching lines @@
     operator >>(other) => 42;
     operator |(other) => 42;
     operator &(other) => 42;
     operator ^(other) => 42;
 
     operator >(other) => true;
     operator >=(other) => true;
     operator <(other) => true;
     operator <=(other) => true;
     operator ==(other) => true;
+    get hashCode => throw "JSNumber.hashCode not implemented.";
 
     abs() => (this is JSInt) ? 42 : 42.0;
     remainder(other) => (this is JSInt) ? 42 : 42.0;
   }
   class JSInt extends JSNumber implements int {
   }
   class JSDouble extends JSNumber implements double {
   }
   class JSNull extends Interceptor {
     bool operator==(other) => identical(null, other);
+    get hashCode => throw "JSNull.hashCode not implemented.";
   }
   class JSBool extends Interceptor implements bool {
   }
   class JSFunction extends Interceptor implements Function {
   }
   class ObjectInterceptor {
   }
   getInterceptor(x) {}
   getNativeInterceptor(x) {}
   var dispatchPropertyName;
   getDispatchProperty(o) {}
   initializeDispatchProperty(f,p,i) {}
   initializeDispatchPropertyCSP(f,p,i) {}
 ''';
 
 const String DEFAULT_CORELIB = r'''
   print(var obj) {}
   abstract class num {}
   abstract class int extends num { }
   abstract class double extends num {
     static var NAN = 0;
     static parse(s) {}
   }
   class bool {}
   class String implements Pattern {}
   class Object {
     operator ==(other) { return true; }
+    get hashCode => throw "Object.hashCode not implemented.";
     String toString() { return null; }
     noSuchMethod(im) { throw im; }
   }
   abstract class StackTrace {}
   class Type {}
   class Function {}
   class List<E> {
     List([length]);
     List.filled(length, element);
   }
@@ skipping 259 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }