Add EfficientLength marker interface to some iterabels.

sdk/lib/utf/utf.dart

Index: sdk/lib/utf/utf.dart
diff --git a/sdk/lib/utf/utf.dart b/sdk/lib/utf/utf.dart
index 229b5aab647bd1b12fc2e8189496533235cafc27..36da078aa1daea966c3c78c28283babbd1e4e171 100644
--- a/sdk/lib/utf/utf.dart
+++ b/sdk/lib/utf/utf.dart
@@ -222,7 +222,7 @@ class Utf16CodeUnitDecoder implements Iterator<int> {
  */
 // TODO(floitsch): Consider removing the extend and switch to implements since
 // that's cheaper to allocate.
-class _ListRange extends IterableBase {
+class _ListRange extends IterableBase implements EfficientLength {
   final List _source;
   final int _offset;
   final int _length;