fix unterminated interpolation expression handling

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 261 matching lines @@
   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.
+   * This method is called when the scanner finds an unterminated
+   * interpolation expression.
    */
   void discardInterpolation() {
-    if (groupingStack.isEmpty) return;
-    BeginToken begin = groupingStack.head;
-    if (begin.kind != STRING_INTERPOLATION_TOKEN) return;
-    unmatchedBeginGroup(begin);
-    groupingStack = groupingStack.tail;
+    while (!groupingStack.isEmpty) {
+      BeginToken beginToken = groupingStack.head;
+      unmatchedBeginGroup(beginToken);
+      groupingStack = groupingStack.tail;
+      if (identical(beginToken.kind, STRING_INTERPOLATION_TOKEN)) break;
+    }
   }
 
   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]
@@ skipping 33 matching lines @@
     //      |
     //     next
     //      v
     //     EOF
     TokenType type = closeBraceInfoFor(begin);
     appendToken(new SyntheticToken(type, tokenStart));
     begin.endGroup = tail;
     appendErrorToken(new UnmatchedToken(begin));
   }
 }