Spelling fixes e to i.

tests/lib/async/zone_bind_callback_unary_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'dart:async';
 
 main() {
   Completer done = new Completer();
@@ skipping 18 matching lines @@
 
   valueToCapture = 499;
   var fun = (x) {
     Expect.identical(forked, Zone.current);
     return x + 99;
   };
   var bound = forked.bindUnaryCallback(fun, runGuarded: false);
   Expect.isFalse(identical(fun, bound));
 
   // It is legal to invoke the callback in a different zone. This is, of course,
-  // etremely discouraged.
+  // extremely discouraged.
   var result = bound(399);
   Expect.equals(498, result);
   Expect.equals(499, restoredValue);
 }