Convert some for-in loops for performance

pkg/analyzer/lib/dart/ast/token.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Defines the tokens that are produced by the scanner, used by the parser, and
  * referenced from the [AST structure](ast.dart).
  */
 library analyzer.dart.ast.token;
 
@@ skipping 342 matching lines @@
   /**
    * Compare the given [tokens] to find the token that appears first in the
    * source being parsed. That is, return the left-most of all of the tokens.
    * The list must be non-`null`, but the elements of the list are allowed to be
    * `null`. Return the token with the smallest offset, or `null` if the list is
    * empty or if all of the elements of the list are `null`.
    */
   static Token lexicallyFirst(List<Token> tokens) {
     Token first = null;
     int offset = -1;
-    for (Token token in tokens) {
+    int length = tokens.length;
+    for (int i = 0; i < length; i++) {
+      Token token = tokens[i];
       if (token != null && (offset < 0 || token.offset < offset)) {
         first = token;
         offset = token.offset;
       }
     }
     return first;
   }
 }
 
 /**
@@ skipping 360 matching lines @@
  */
 class _EndOfFileTokenType extends TokenType {
   /**
    * Initialize a newly created token.
    */
   const _EndOfFileTokenType() : super._('EOF', TokenClass.NO_CLASS, '');
 
   @override
   String toString() => '-eof-';
 }