improve fasta parser export declaration recovery

pkg/analyzer/lib/src/fasta/ast_builder.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 file.
 
 library fasta.analyzer.ast_builder;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart' show AstFactory;
 import 'package:analyzer/dart/ast/standard_ast_factory.dart' as standard;
 import 'package:analyzer/dart/ast/token.dart' as analyzer show Token;
@@ skipping 1910 matching lines @@
   }
 
   @override
   void addCompileTimeErrorFromMessage(FastaMessage message) {
     FastaCode code = message.code;
     switch (code.analyzerCode) {
       case "EXPECTED_TYPE_NAME":
         errorReporter?.reportErrorForOffset(
             ParserErrorCode.EXPECTED_TYPE_NAME, message.charOffset, 1);
         return;
+      case "EXPECTED_STRING_LITERAL":
+        errorReporter?.reportErrorForOffset(
+            ParserErrorCode.EXPECTED_STRING_LITERAL, message.charOffset, 1);
+        return;
       default:
       // fall through
     }
     library.addCompileTimeError(message.charOffset, message.message,
         fileUri: message.uri);
   }
 }
 
 /// Data structure placed on the stack to represent a class body.
 ///
@@ skipping 93 matching lines @@
       } else if (identical('var', s)) {
         finalConstOrVarKeyword = token;
       } else if (identical('covariant', s)) {
         covariantKeyword = token;
       } else {
         internalError('Unhandled modifier: $s');
       }
     }
   }
 }