improve fasta export directive 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 f7e2e1f90c1ca2e2ecc07d131512ef7aaa3a7661..9560cd4aa3454ee6af4fcd7a7beee9bf97405d01 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -20,6 +20,8 @@ import '../../scanner/token.dart'
         EQUALITY_PRECEDENCE,
         POSTFIX_PRECEDENCE,
         RELATIONAL_PRECEDENCE,
+        SyntheticStringToken,
+        SyntheticToken,
         TokenType;
 
 import '../scanner/token.dart' show isUserDefinableOperator;
@@ -356,13 +358,12 @@ class Parser {
     Token exportKeyword = token;
     listener.beginExport(exportKeyword);
     assert(optional('export', token));
-    token = parseLiteralStringOrRecoverExpression(token.next);
+    token = ensureParseLiteralString(token.next);
     token = parseConditionalUris(token);
     token = parseCombinators(token);
-    Token semicolon = token;
-    token = expect(';', token);
+    Token semicolon = ensureSemicolon(token);
     listener.endExport(exportKeyword, semicolon);
-    return token;
+    return semicolon.next;
   }
 
   Token parseCombinators(Token token) {
@@ -1922,6 +1923,24 @@ class Parser {
     return token;
   }
 
+  Token ensureParseLiteralString(Token token) {
+    if (!identical(token.kind, STRING_TOKEN)) {

[ahe, 2017/08/21 12:51:37]

Don't need to use identical on int.

[danrubel, 2017/08/21 14:51:02]

Fair enough. I was just following what you already did on line 1945 below.

+      reportRecoverableErrorWithToken(token, fasta.templateExpectedString);
+      token = listener.rewriter.insertTokenBefore(

[ahe, 2017/08/21 12:51:37]

Something is off here. If the parser does the rewriting, it doesn't make sense
that the listener holds the rewriter.

[danrubel, 2017/08/21 14:51:02]

Yeah. I had the parser holding onto the rewriter but was asked to move it into
the listener. It makes sense for the parser to do the recovery because otherwise
various listeners will recover differently, which will produce different error
messages and different generated code for different listeners and confuse the
user.

+          new SyntheticStringToken(TokenType.STRING, '""', token.charOffset, 0),
+          token);
+    }
+    return parseLiteralString(token);
+  }
+
+  Token ensureSemicolon(Token token) {
+    if (optional(';', token)) return token;
+    reportRecoverableError(
+        token, fasta.templateExpectedButGot.withArguments(';'));
+    return listener.rewriter.insertTokenBefore(
+        new SyntheticToken(TokenType.SEMICOLON, token.charOffset), token);
+  }
+
   Token parseLiteralStringOrRecoverExpression(Token token) {
     if (identical(token.kind, STRING_TOKEN)) {
       return parseLiteralString(token);