Properly parse [].

lib/compiler/implementation/scanner/array_based_scanner.dart

Index: lib/compiler/implementation/scanner/array_based_scanner.dart
diff --git a/lib/compiler/implementation/scanner/array_based_scanner.dart b/lib/compiler/implementation/scanner/array_based_scanner.dart
index 4000cc5b103d2e903b5d163639bcdf1078626213..c3dfb5fc88c9dc003feef2910cb4624222f45dfa 100644
--- a/lib/compiler/implementation/scanner/array_based_scanner.dart
+++ b/lib/compiler/implementation/scanner/array_based_scanner.dart
@@ -74,6 +74,10 @@ class ArrayBasedScanner<S extends SourceString> extends AbstractScanner<S> {
     return tokens.next;
   }
 
+  Token previousToken() {
+    return tail;
+  }
+
   void addToCharOffset(int offset) {
     extraCharOffset += offset;
   }