improve fasta parser export declaration recovery

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

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index 02bee28ffd2e7a27d90d854c873a7754b8e2dd98..dd76274961cc3f9c754b1b57ed5a99f9d4fa0f15 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -77,6 +77,7 @@ import '../../scanner/token.dart'
         EQUALITY_PRECEDENCE,
         POSTFIX_PRECEDENCE,
         RELATIONAL_PRECEDENCE,
+        SyntheticStringToken,
         TokenType;
 
 import '../scanner/token.dart' show isUserDefinableOperator;
@@ -114,6 +115,7 @@ import 'async_modifier.dart' show AsyncModifier;
 import 'listener.dart' show Listener;
 
 import 'identifier_context.dart' show IdentifierContext;
+import 'package:front_end/src/fasta/parser/token_stream_rewriter.dart';
 
 /// Returns true if [token] is the symbol or keyword [value].
 bool optional(String value, Token token) {
@@ -310,6 +312,8 @@ class Parser {
 
   bool mayParseFunctionExpressions = true;
 
+  TokenStreamRewriter _rewriter;
+
   /// Represents parser state: what asynchronous syntax is allowed in the
   /// function being currently parsed. In rare situations, this can be set by
   /// external clients, for example, to parse an expression outside a function.
@@ -330,6 +334,7 @@ class Parser {
   bool get inPlainSync => asyncState == AsyncModifier.Sync;
 
   Token parseUnit(Token token) {
+    _rewriter = new TokenStreamRewriter(token);
     listener.beginCompilationUnit(token);
     int count = 0;
     while (!identical(token.kind, EOF_TOKEN)) {
@@ -459,7 +464,7 @@ class Parser {
     Token exportKeyword = token;
     listener.beginExport(exportKeyword);
     assert(optional('export', token));
-    token = parseLiteralStringOrRecoverExpression(token.next);
+    token = parseLiteralString(ensureLiteralString(token.next));
     token = parseConditionalUris(token);
     token = parseCombinators(token);
     Token semicolon = token;
@@ -1085,6 +1090,17 @@ class Parser {
     return token.next;
   }
 
+  Token ensureLiteralString(Token token) {
+    if (!identical(token.kind, STRING_TOKEN)) {
+      print('>>> ensureLiteralString');
+      reportRecoverableErrorCodeWithToken(token, codeExpectedString);
+      token = _rewriter.insertTokenBefore(
+          new SyntheticStringToken(TokenType.STRING, '""', token.offset, 0),
+          token);
+    }
+    return token;
+  }
+
   Token expect(String string, Token token) {
     if (!identical(string, token.stringValue)) {
       return reportUnrecoverableErrorCodeWithString(