improve fasta interpolation recovery

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

 // Copyright (c) 2011, 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.scanner.array_based_scanner;
 
 import 'error_token.dart' show ErrorToken, UnmatchedToken;
 
 import '../../scanner/token.dart'
     show
@@ skipping 104 matching lines @@
    *
    * This method is used by the scanners to track line breaks and create the
    * [lineStarts] map.
    */
   void lineFeedInMultiline() {
     lineStarts.add(stringOffset + 1);
   }
 
   /**
    * Appends a token that begins a new group, represented by [type].
-   * Group begin tokens are '{', '(', '[' and '${'.
+   * Group begin tokens are '{', '(', '[', '<' and '${'.
    */
   void appendBeginGroup(TokenType type) {
     Token token = new BeginTokenWithComment(type, tokenStart, comments);
     appendToken(token);
 
     // { [ ${ cannot appear inside a type parameters / arguments.
     if (!identical(type.kind, LT_TOKEN) &&
         !identical(type.kind, OPEN_PAREN_TOKEN)) {
       discardOpenLt();
     }
@@ skipping 114 matching lines @@
    * something which cannot possibly be part of a type parameter/argument
    * list, like the '=' in the above example.
    */
   void discardOpenLt() {
     while (!groupingStack.isEmpty &&
         identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 
+  /**
+   * This method is called to discard '${' from the "grouping" stack.
+   *
+   * This method is called when the scanner finds the end of a string
+   * or an unterminated string.

[ahe, 2017/05/31 13:15:48]

I think this is only called when it finds an unterminated string interpolation.

[danrubel, 2017/06/01 19:02:46]

Right. Fixed in https://codereview.chromium.org/2914213003/

+   */
+  void discardInterpolation() {
+    if (groupingStack.isEmpty) return;
+    BeginToken begin = groupingStack.head;
+    if (begin.kind != STRING_INTERPOLATION_TOKEN) return;
+    unmatchedBeginGroup(begin);
+    groupingStack = groupingStack.tail;

[ahe, 2017/05/31 13:15:48]

What happens in this case:

"${([{

[danrubel, 2017/06/01 19:02:46]

Hmmm... good point. Fixed in https://codereview.chromium.org/2914213003/

+  }
+
   void unmatchedBeginGroup(BeginToken begin) {
     // We want to ensure that unmatched BeginTokens are reported as
     // errors.  However, the diet parser assumes that groups are well-balanced
     // and will never look at the endGroup token.  This is a nice property that
     // allows us to skip quickly over correct code. By inserting an additional
     // synthetic token in the stream, we can keep ignoring endGroup tokens.
     //
     // [begin] --next--> [tail]
     // [begin] --endG--> [synthetic] --next--> [next] --next--> [tail]
     //
@@ skipping 31 matching lines @@
     //      |
     //     next
     //      v
     //     EOF
     TokenType type = closeBraceInfoFor(begin);
     appendToken(new SyntheticToken(type, tokenStart));
     begin.endGroup = tail;
     appendErrorToken(new UnmatchedToken(begin));
   }
 }