Move Iterable implementation to collection.

runtime/lib/immutable_map.dart

Index: runtime/lib/immutable_map.dart
diff --git a/runtime/lib/immutable_map.dart b/runtime/lib/immutable_map.dart
index 82f54580e4490a66e346b3f206475b8a93fdb8c8..3ca9ed53d861e96c2187e2612a9ad4a89c25f5ec 100644
--- a/runtime/lib/immutable_map.dart
+++ b/runtime/lib/immutable_map.dart
@@ -82,7 +82,7 @@ class ImmutableMap<K, V> implements Map<K, V> {
   }
 }
 
-class _ImmutableMapKeyIterable<E> extends Iterable<E> {
+class _ImmutableMapKeyIterable<E> extends IterableBase<E> {
   final ImmutableMap _map;
   _ImmutableMapKeyIterable(this._map);
 
@@ -91,7 +91,7 @@ class _ImmutableMapKeyIterable<E> extends Iterable<E> {
   }
 }
 
-class _ImmutableMapValueIterable<E> extends Iterable<E> {
+class _ImmutableMapValueIterable<E> extends IterableBase<E> {
   final ImmutableMap _map;
   _ImmutableMapValueIterable(this._map);