Add EfficientLength marker interface to some iterabels.

pkg/intl/lib/number_format.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.
 
 part of intl;
 /**
  * Provides the ability to format a number in a locale-specific way. The
  * format is specified as a pattern using a subset of the ICU formatting
  * patterns.
  *
@@ skipping 702 matching lines @@
 class _StringIterator implements Iterator<String> {
   String input;
   var index = -1;
   inBounds(i) => i >= 0 && i < input.length;
   _StringIterator(this.input);
   String get current => inBounds(index) ? input[index] : null;
 
   bool moveNext() => inBounds(++index);
   String get peek => inBounds(index + 1) ? input[index + 1] : null;
   Iterator<String> get iterator => this;
-}
+}