Add EfficientLength marker interface to some iterabels.

pkg/intl/lib/number_format.dart

Index: pkg/intl/lib/number_format.dart
diff --git a/pkg/intl/lib/number_format.dart b/pkg/intl/lib/number_format.dart
index bdc5e46aed2030535c020500dba0991315853edd..a7acbafa820abdd500b6bf4c2548af0d56124333 100644
--- a/pkg/intl/lib/number_format.dart
+++ b/pkg/intl/lib/number_format.dart
@@ -720,4 +720,4 @@ class _StringIterator implements Iterator<String> {
   bool moveNext() => inBounds(++index);
   String get peek => inBounds(index + 1) ? input[index + 1] : null;
   Iterator<String> get iterator => this;
-}
+}