Warn about overriding operator== but not hashCode.

dart/sdk/lib/_internal/compiler/implementation/warnings.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.
 
 part of dart2js;
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
@@ skipping 415 matching lines @@
       '"_".');
 
   static const INVALID_SYMBOL = const MessageKind('''
 Error: "#{value}" is not a valid Symbol name because is not:
  * an empty String,
  * a user defined operator,
  * a qualified non-private identifier optionally followed by "=", or
  * a qualified non-private identifier followed by "." and a user-defined operator.''');
 
   static const AMBIGUOUS_REEXPORT = const MessageKind(
-    'Info: "#{element}" is (re)exported by multiple libraries.');
+      'Info: "#{element}" is (re)exported by multiple libraries.');
 
   static const AMBIGUOUS_LOCATION = const MessageKind(
-    'Info: "#{element}" is defined here.');
+      'Info: "#{element}" is defined here.');
 
   static const IMPORTED_HERE = const MessageKind(
-    'Info: "#{element}" is imported here.');
+      'Info: "#{element}" is imported here.');
+
+  static const OVERRIDE_EQUALS_NOT_HASH_CODE = const MessageKind(
+      'Warning: The class "#{class}" overrides "operator==", '
+      'but not "get hashCode".');
 
   static const COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");
 
   static const PLEASE_REPORT_THE_CRASH = const MessageKind('''
 The compiler is broken.
 
 When compiling the above element, the compiler crashed. It is not
 possible to tell if this is caused by a problem in your program or
 not. Regardless, the compiler should not crash.
@@ skipping 176 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }