Add compiler error when instantiating abstract class

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 11549)
+++ runtime/vm/object.cc	(working copy)
@@ -1837,7 +1837,12 @@
   raw_ptr()->state_bits_ = InterfaceBit::update(true, bits);
 }
 
+void Class::set_is_abstract() const {
+  uword bits = raw_ptr()->state_bits_;
+  raw_ptr()->state_bits_ = AbstractBit::update(true, bits);
+}
 
+
 void Class::set_is_const() const {
   uword bits = raw_ptr()->state_bits_;
   raw_ptr()->state_bits_ = ConstBit::update(true, bits);