Use named arguments for messages.

sdk/lib/_internal/compiler/implementation/util/util.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.
 
 library org_dartlang_compiler_util;
 
 import 'util_implementation.dart';
 import 'characters.dart';
 
 part 'link.dart';
 
 /**
  * Tagging interface for classes from which source spans can be generated.
  */
 // TODO(johnniwinther): Find a better name.
 // TODO(ahe): How about "Bolt"?
 abstract class Spannable {}
 
+class _SpannableSentinel implements Spannable {
+  final String name;
+
+  const _SpannableSentinel(this.name);
+
+  String toString() => name;
+}
+
+const Spannable CURRENT_ELEMENT_SPANNABLE =
+    const _SpannableSentinel("Current element");
+
 class SpannableAssertionFailure {
   final Spannable node;
   final String message;
   SpannableAssertionFailure(this.node, this.message);
 
   String toString() => 'Compiler crashed: $message.';
 }
 
 /// Writes the characters of [string] on [buffer].  The characters
 /// are escaped as suitable for JavaScript and JSON.  [buffer] is
@@ skipping 60 matching lines @@
   for (int i = 0; i < string.length; i++) {
     int code = string.charCodeAt(i);
     if (code < 0x20 || code == $DEL || code == $DQ || code == $LS ||
         code == $PS || code == $BACKSLASH || code >= 0x80) {
       writeEscapedOn(string, buffer);
       return;
     }
   }
   buffer.add(string);
 }