Implement await for statement

runtime/vm/object_store.h

Index: runtime/vm/object_store.h
===================================================================
--- runtime/vm/object_store.h	(revision 41324)
+++ runtime/vm/object_store.h	(working copy)
@@ -124,6 +124,11 @@
     completer_class_ = value.raw();
   }
 
+  RawClass* stream_iterator_class() const { return stream_iterator_class_; }
+  void set_stream_iterator_class(const Class& value) {
+    stream_iterator_class_ = value.raw();
+  }
+
   RawClass* one_byte_string_class() const { return one_byte_string_class_; }
   void set_one_byte_string_class(const Class& value) {
     one_byte_string_class_ = value.raw();
@@ -447,6 +452,7 @@
   RawType* string_type_;
   RawClass* future_class_;
   RawClass* completer_class_;
+  RawClass* stream_iterator_class_;
   RawClass* one_byte_string_class_;
   RawClass* two_byte_string_class_;
   RawClass* external_one_byte_string_class_;