fix $this in string interpolation

frog/parser.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.
 
 // TODO(jimhug): Error recovery needs major work!
 /**
  * A simple recursive descent parser for the dart language.
  *
  * This parser is designed to be more permissive than the official
  * Dart grammar.  It is expected that many grammar errors would be
@@ skipping 1076 matching lines @@
           startQuote = endQuote = text[0];
         }
         text = text.substring(0, text.length-1) + endQuote; // fix trailing $
       } else {
         text = startQuote + text.substring(0, text.length-1) + endQuote;
       }
       lits.add(makeStringLiteral(text, token.span));
       if (_maybeEat(TokenKind.LBRACE)) {
         lits.add(expression());
         _eat(TokenKind.RBRACE);
+      } else if (_maybeEat(TokenKind.THIS)) {
+        lits.add(new ThisExpression(_previousToken.span));
       } else {
         var id = identifier();
         lits.add(new VarExpression(id, id.span));
       }
     }
     var tok = _next();
     if (tok.kind != TokenKind.STRING) {
       _errorExpected('interpolated string');
     }
     var text = startQuote + tok.text;
@@ skipping 570 matching lines @@
 class IncompleteSourceException implements Exception {
   final Token token;
 
   IncompleteSourceException(this.token);
 
   String toString() {
     if (token.span == null) return 'Unexpected $token';
     return token.span.toMessageString('Unexpected $token');
   }
 }