improve fasta export directive recovery

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

Index: pkg/front_end/lib/src/fasta/parser/listener.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/listener.dart b/pkg/front_end/lib/src/fasta/parser/listener.dart
index cf593707ba667256f50549309facba3f41fc13df..bec0c9ad2a4bf88be9f3f0fa8b757b418d227259 100644
--- a/pkg/front_end/lib/src/fasta/parser/listener.dart
+++ b/pkg/front_end/lib/src/fasta/parser/listener.dart
@@ -20,6 +20,8 @@ import 'member_kind.dart' show MemberKind;
 
 import 'parser_error.dart' show ParserError;
 
+import 'token_stream_rewriter.dart';
+
 /// A parser event listener that does nothing except throw exceptions
 /// on parser errors.
 ///
@@ -34,6 +36,23 @@ import 'parser_error.dart' show ParserError;
 class Listener {
   final List<ParserError> recoverableErrors = <ParserError>[];
 
+  /// The first token in the parse stream and used during parser recovery.
+  /// This is automatically set by the [beginCompilationUnit] event,
+  /// but must be manually set when parsing anything smaller.
+  Token firstToken;
+
+  /// A rewriter for inserting synthetic tokens.
+  /// Access using [rewriter] for lazy initialization.
+  TokenStreamRewriter _tokenRewriter;
+
+  TokenStreamRewriter get rewriter {
+    assert(firstToken != null, 'firstToken must be set for parser recovery');
+    _tokenRewriter ??= new TokenStreamRewriter(firstToken);
+    return _tokenRewriter;
+  }
+
+  Listener();
+
   Uri get uri => null;
 
   void logEvent(String name) {}
@@ -112,7 +131,9 @@ class Listener {
     logEvent("Combinators");
   }
 
-  void beginCompilationUnit(Token token) {}
+  void beginCompilationUnit(Token token) {
+    firstToken = token;
+  }
 
   void endCompilationUnit(int count, Token token) {
     logEvent("CompilationUnit");