Use FastaMessage instead of String. Part 1.

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

 // Copyright (c) 2017, 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 licenset hat can be found in the LICENSE file.
 
 library fasta.scanner.recover;
 
 import '../../scanner/token.dart' show TokenType;
 
 import '../fasta_codes.dart'
     show
-        FastaCode,
+        Code,
         codeAsciiControlCharacter,
         codeEncoding,
         codeExpectedHexDigit,
         codeMissingExponent,
         codeNonAsciiIdentifier,
         codeNonAsciiWhitespace,
         codeUnexpectedDollarInString,
         codeUnmatchedToken,
         codeUnterminatedComment,
         codeUnterminatedString;
@@ skipping 143 matching lines @@
         if (errorTail == null) {
           error = next;
         } else {
           errorTail.next = next;
           next.previous = errorTail;
         }
         errorTail = next;
         next = next.next;
       } while (next is ErrorToken && first.errorCode == next.errorCode);
 
-      FastaCode code = first.errorCode;
+      Code code = first.errorCode;
       if (code == codeEncoding ||
           code == codeNonAsciiWhitespace ||
           code == codeAsciiControlCharacter) {
         current = errorTail.next;
       } else if (code == codeNonAsciiIdentifier) {
         current = recoverIdentifier(first);
         assert(current.next != null);
       } else if (code == codeMissingExponent) {
         current = recoverExponent();
         assert(current.next != null);
@@ skipping 52 matching lines @@
 
 String closeBraceFor(String openBrace) {
   return const {
     '(': ')',
     '[': ']',
     '{': '}',
     '<': '>',
     r'${': '}',
   }[openBrace];
 }