Properly parse [].

lib/compiler/implementation/scanner/scanner.dart

 // Copyright (c) 2012, 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.
 
 interface Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
  */
 class AbstractScanner<T extends SourceString> implements Scanner {
   abstract int advance();
   abstract int nextByte();
   abstract int peek();
   abstract int select(int choice, PrecedenceInfo yes, PrecedenceInfo no);
   abstract void appendPrecedenceToken(PrecedenceInfo info);
   abstract void appendStringToken(PrecedenceInfo info, String value);
   abstract void appendByteStringToken(PrecedenceInfo info, T value);
   abstract void appendKeywordToken(Keyword keyword);
   abstract void appendWhiteSpace(int next);
   abstract void appendEofToken();
   abstract T asciiString(int start, int offset);
   abstract T utf8String(int start, int offset);
   abstract Token firstToken();
+  abstract Token previousToken();
   abstract void beginToken();
   abstract void addToCharOffset(int offset);
   abstract int get charOffset();
   abstract int get byteOffset();
   abstract void appendBeginGroup(PrecedenceInfo info, String value);
   abstract int appendEndGroup(PrecedenceInfo info, String value, int openKind);
   abstract void appendGt(PrecedenceInfo info, String value);
   abstract void appendGtGt(PrecedenceInfo info, String value);
   abstract void appendGtGtGt(PrecedenceInfo info, String value);
   abstract void discardOpenLt();
@@ skipping 200 matching lines @@
     } else {
       appendPrecedenceToken(TILDE_INFO);
       return next;
     }
   }
 
   int tokenizeOpenSquareBracket(int next) {
     // [ [] []=
     next = advance();
     if (next === $CLOSE_SQUARE_BRACKET) {
-      return select($EQ, INDEX_EQ_INFO, INDEX_INFO);
-    } else {
-      appendBeginGroup(OPEN_SQUARE_BRACKET_INFO, "[");
-      return next;
+      Token token = previousToken();
+      if (token is KeywordToken && token.value == Keyword.OPERATOR) {
+        return select($EQ, INDEX_EQ_INFO, INDEX_INFO);
+      }
     }
+    appendBeginGroup(OPEN_SQUARE_BRACKET_INFO, "[");
+    return next;
   }
 
   int tokenizeCaret(int next) {
     // ^ ^=
     return select($EQ, CARET_EQ_INFO, CARET_INFO);
   }
 
   int tokenizeBar(int next) {
     // | || |=
     next = advance();
@@ skipping 497 matching lines @@
                                       charOffset);
   }
 }
 
 class MalformedInputException {
   final String message;
   final position;
   MalformedInputException(this.message, this.position);
   toString() => message;
 }