Properly parse [].

lib/compiler/implementation/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 SourceString> extends AbstractScanner<S> {
   int get charOffset() => byteOffset + extraCharOffset;
   final Token tokens;
   Token tail;
   int tokenStart;
   int byteOffset;
@@ skipping 56 matching lines @@
   }
 
   void beginToken() {
     tokenStart = charOffset;
   }
 
   Token firstToken() {
     return tokens.next;
   }
 
+  Token previousToken() {
+    return tail;
+  }
+
   void addToCharOffset(int offset) {
     extraCharOffset += offset;
   }
 
   void appendWhiteSpace(int next) {
     // Do nothing, we don't collect white space.
   }
 
   void appendBeginGroup(PrecedenceInfo info, String value) {
     Token token = new BeginGroupToken(info, value, tokenStart);
@@ skipping 75 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();
 }