Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/scanner/token.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 scanner;
 
 const int EOF_TOKEN = 0;
 
 const int KEYWORD_TOKEN = $k;
 const int IDENTIFIER_TOKEN = $a;
@@ skipping 151 matching lines @@
       // instead of the size of the length of the error message.
       return 1;
     } else {
       return value.length;
     }
   }
 
   int get hashCode => computeHashCode(charOffset, info, value);
 }
 
+/// A pair of tokens marking the beginning and the end of a span. Use for error
+/// reporting.
+class TokenPair implements Spannable {
+  final Token begin;
+  final Token end;
+
+  TokenPair(this.begin, this.end);
+}
+
 /**
  * A [SymbolToken] represents the symbol in its precendence info.
  * Also used for end of file with EOF_INFO.
  */
 class SymbolToken extends Token {
 
   final PrecedenceInfo info;
 
   SymbolToken(this.info, int charOffset) : super(charOffset);
 
@@ skipping 531 matching lines @@
 
 const PrecedenceInfo STRING_INTERPOLATION_IDENTIFIER_INFO =
   const PrecedenceInfo('\$', 0,
                        STRING_INTERPOLATION_IDENTIFIER_TOKEN);
 
 const PrecedenceInfo HEXADECIMAL_INFO =
   const PrecedenceInfo('hexadecimal', 0, HEXADECIMAL_TOKEN);
 
 const PrecedenceInfo COMMENT_INFO =
   const PrecedenceInfo('comment', 0, COMMENT_TOKEN);