Check that const-map keys don't override equals.

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;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 538 matching lines @@
 
   static const MessageKind NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
       "Not a compile-time constant.");
 
   static const MessageKind CYCLIC_COMPILE_TIME_CONSTANTS = const MessageKind(
       "Cycle in the compile-time constant computation.");
 
   static const MessageKind CONSTRUCTOR_IS_NOT_CONST = const MessageKind(
       "Constructor is not a 'const' constructor.");
 
-  static const MessageKind KEY_NOT_A_STRING_LITERAL = const MessageKind(
-      "Map-literal key not a string literal.");
+  static const MessageKind CONST_MAP_KEY_OVERRIDES_EQUALS =
+      const MessageKind(
+          "Const-map key type '#{type}' overrides 'operator =='.");
 
   static const MessageKind NO_SUCH_LIBRARY_MEMBER = const MessageKind(
       "'#{libraryName}' has no member named '#{memberName}'.");
 
   static const MessageKind CANNOT_INSTANTIATE_TYPEDEF = const MessageKind(
       "Cannot instantiate typedef '#{typedefName}'.");
 
   static const MessageKind REQUIRED_PARAMETER_WITH_DEFAULT = const MessageKind(
       "Non-optional parameters can't have a default value.",
       howToFix:
@@ skipping 1319 matching lines @@
     return computeMessage();
   }
 
   bool operator==(other) {
     if (other is !Message) return false;
     return (kind == other.kind) && (toString() == other.toString());
   }
 
   int get hashCode => throw new UnsupportedError('Message.hashCode');
 }