Add EfficientLength marker interface to some iterabels.

sdk/lib/_internal/lib/constant_map.dart

Index: sdk/lib/_internal/lib/constant_map.dart
diff --git a/sdk/lib/_internal/lib/constant_map.dart b/sdk/lib/_internal/lib/constant_map.dart
index 7febd185ff96805409dae7be7db215348c854c18..c8bbe6d610c49c7cf7c41a2561049a90b03dc224 100644
--- a/sdk/lib/_internal/lib/constant_map.dart
+++ b/sdk/lib/_internal/lib/constant_map.dart
@@ -23,7 +23,8 @@ abstract class ConstantMap<K, V> implements Map<K, V> {
 
 // This class has no constructor. This is on purpose since the instantiation
 // is shortcut by the compiler.
-class ConstantStringMap<K, V> extends ConstantMap<K, V> {
+class ConstantStringMap<K, V> extends ConstantMap<K, V>
+                              implements EfficientLength {
   final int length;
   // A constant map is backed by a JavaScript object.
   final _jsObject;