improve fasta interpolation recovery

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

Index: pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
index a05e0f44798bb38195266815410628cd9f109f15..1eb5ffb39151ac09fb1dfd829ca73ee356586dcb 100644
--- a/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/array_based_scanner.dart
@@ -122,7 +122,7 @@ abstract class ArrayBasedScanner extends AbstractScanner {
 
   /**
    * 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);
@@ -257,6 +257,20 @@ abstract class ArrayBasedScanner extends AbstractScanner {
     }
   }
 
+  /**
+   * 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