Mix in [TreeElementMixin] only on nodes that need it.

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 1459 matching lines @@
   static const MessageKind WRONG_ARGUMENT_FOR_JS_INTERCEPTOR_CONSTANT =
       const MessageKind(
       "Argument for 'JS_INTERCEPTOR_CONSTANT' must be a type constant.");
 
   static const MessageKind EXPECTED_IDENTIFIER_NOT_RESERVED_WORD =
       const MessageKind(
           "'#{keyword}' is a reserved word and can't be used here.",
           howToFix: "Try using a different name.",
           examples: const ["do() {} main() {}"]);
 
+  static const MessageKind  NAMED_FUNCTION_EXPRESSION =
+      const MessageKind("Function expression '#{name}' cannot be named.",
+          howToFix: "Try removing the name.",
+          examples: const ["main() { var f = func() {}; }"]);
+
   static const MessageKind UNUSED_METHOD = const MessageKind(
       "The method '#{name}' is never called.",
       howToFix: "Consider deleting it.",
       examples: const ["deadCode() {} main() {}"]);
 
   static const MessageKind UNUSED_CLASS = const MessageKind(
       "The class '#{name}' is never used.",
       howToFix: "Consider deleting it.",
       examples: const ["class DeadCode {} main() {}"]);
 
@@ skipping 617 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }