Port parser and scanner fixes from rasta branch.

pkg/front_end/lib/src/fasta/scanner.dart

 // Copyright (c) 2016, 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.md file.
 
 library fasta.scanner;
 
+import 'dart:convert' show
+    UNICODE_REPLACEMENT_CHARACTER_RUNE;
+
 import 'scanner/token.dart' show
     Token;
 
 import 'scanner/utf8_bytes_scanner.dart' show
     Utf8BytesScanner;
 
 import 'scanner/recover.dart' show
     defaultRecoveryStrategy;
 
 export 'scanner/token.dart' show
     BeginGroupToken,
-    ErrorToken,
     KeywordToken,
     StringToken,
     SymbolToken,
     Token,
     isBinaryOperator,
     isMinusOperator,
     isTernaryOperator,
     isUnaryOperator,
     isUserDefinableOperator;
 
+export 'scanner/error_token.dart' show
+    ErrorToken,
+    buildUnexpectedCharacterToken;
+
 export 'scanner/token_constants.dart' show
     EOF_TOKEN;
 
 export 'scanner/utf8_bytes_scanner.dart' show
     Utf8BytesScanner;
 
 export 'scanner/string_scanner.dart' show
     StringScanner;
 
 export 'scanner/keyword.dart' show
     Keyword;
 
+const int unicodeReplacementCharacter = UNICODE_REPLACEMENT_CHARACTER_RUNE;
+
 typedef Token Recover(List<int> bytes, Token tokens, List<int> lineStarts);
 
 abstract class Scanner {
   /// Returns true if an error occured during [tokenize].
   bool get hasErrors;
 
   List<int> get lineStarts;
 
   Token tokenize();
 }
@@ skipping 12 matching lines @@
   }
   Scanner scanner =
       new Utf8BytesScanner(bytes, includeComments: includeComments);
   Token tokens = scanner.tokenize();
   if (scanner.hasErrors) {
     recover ??= defaultRecoveryStrategy;
     tokens = recover(bytes, tokens, scanner.lineStarts);
   }
   return new ScannerResult(tokens, scanner.lineStarts);
 }