Support symbol literals.

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 597 matching lines @@
 Hint: Using "new #{name}" may result in larger output.
 Use "const #{name}" if possible.''');
 
   static const MessageKind STRING_EXPECTED = const MessageKind(
       'Error: Expected a "String", but got an instance of "#{type}".');
 
   static const MessageKind PRIVATE_IDENTIFIER = const MessageKind(
       'Error: "#{value}" is not a valid Symbol name because it starts with '
       '"_".');
 
+  static const MessageKind UNSUPPORTED_LITERAL_SYMBOL = const MessageKind(
+      'Internal Error: Symbol literal "##{value}" is currently unsupported.');
+
   static const MessageKind 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 MessageKind AMBIGUOUS_REEXPORT = const MessageKind(
       'Info: "#{element}" is (re)exported by multiple libraries.');
@@ skipping 394 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }