Fix NPE in ArrayBasedScanner

dart/frog/leg/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.
 
 class ArrayBasedScanner<S> extends AbstractScanner<S> {
   int get charOffset() => byteOffset + extraCharOffset;
   final Token tokens;
   Token tail;
   int tokenStart;
   int byteOffset;
@@ skipping 79 matching lines @@
            !groupingStack.isEmpty() &&
            groupingStack.head.kind === LT_TOKEN) {
       groupingStack = groupingStack.tail;
     }
     groupingStack = groupingStack.prepend(token);
   }
 
   int appendEndGroup(PrecedenceInfo info, String value, int openKind) {
     assert(openKind !== LT_TOKEN);
     appendStringToken(info, value);
+    discardOpenLt();
     if (groupingStack.isEmpty()) {
       return advance();
     }
-    discardOpenLt();
     BeginGroupToken begin = groupingStack.head;
     if (begin.kind !== openKind) {
       if (openKind !== OPEN_CURLY_BRACKET_TOKEN ||
           begin.kind !== STRING_INTERPOLATION_TOKEN) {
         // Not ending string interpolation.
         throw new MalformedInputException('Unmatched ${begin.stringValue}',
                                           begin);
       }
       // We're ending an interpolated expression.
       begin.endGroup = tail;
@@ skipping 48 matching lines @@
 
   void discardOpenLt() {
     while (!groupingStack.isEmpty() && groupingStack.head.kind === LT_TOKEN) {
       groupingStack = groupingStack.tail;
     }
   }
 
   // TODO(ahe): make class abstract instead of adding an abstract method.
   abstract peek();
 }