Convert some for-in loops for performance

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

Index: pkg/analyzer/lib/dart/ast/token.dart
diff --git a/pkg/analyzer/lib/dart/ast/token.dart b/pkg/analyzer/lib/dart/ast/token.dart
index 77226be42e50b91859abb0144d7411b36eb6d9ee..14ba3f91ba20e449de219c7f44a5bd8543d4757f 100644
--- a/pkg/analyzer/lib/dart/ast/token.dart
+++ b/pkg/analyzer/lib/dart/ast/token.dart
@@ -360,7 +360,9 @@ abstract class Token {
   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;