Diagnose unterminated "grouping" characters.

dart/frog/leg/scanner/array_based_scanner.dart

Index: dart/frog/leg/scanner/array_based_scanner.dart
diff --git a/dart/frog/leg/scanner/array_based_scanner.dart b/dart/frog/leg/scanner/array_based_scanner.dart
index 1a49c2e118c39e623a697673b4df15e5c068c27b..a3e8ca31a068b1418faa83a87994943ed2320c9e 100644
--- a/dart/frog/leg/scanner/array_based_scanner.dart
+++ b/dart/frog/leg/scanner/array_based_scanner.dart
@@ -58,6 +58,11 @@ class ArrayBasedScanner<S> extends AbstractScanner<S> {
     tail = tail.next;
     // EOF points to itself so there's always infinite look-ahead.
     tail.next = tail;
+    if (!groupingStack.isEmpty()) {
+      BeginGroupToken begin = groupingStack.head;
+      throw new MalformedInputException('Unmatched ${begin.stringValue}',
+                                        begin);
+    }
   }
 
   void beginToken() {
@@ -100,7 +105,8 @@ class ArrayBasedScanner<S> extends AbstractScanner<S> {
       if (openKind !== OPEN_CURLY_BRACKET_TOKEN ||
           begin.kind !== STRING_INTERPOLATION_TOKEN) {
         // Not ending string interpolation.
-        throw new MalformedInputException('Unmatched ${begin.stringValue}');
+        throw new MalformedInputException('Unmatched ${begin.stringValue}',
+                                          begin);
       }
       // We're ending an interpolated expression.
       begin.endGroup = tail;