Add compiler error when instantiating abstract class

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 11549)
+++ runtime/vm/object.h	(working copy)
@@ -648,6 +648,11 @@
   }
   void set_is_interface() const;
 
+  bool is_abstract() const {
+    return AbstractBit::decode(raw_ptr()->state_bits_);
+  }
+  void set_is_abstract() const;
+
   bool is_finalized() const {
     return StateBits::decode(raw_ptr()->state_bits_) == RawClass::kFinalized;
   }
@@ -716,11 +721,13 @@
   enum {
     kConstBit = 1,
     kInterfaceBit = 2,
-    kStateTagBit = 3,
+    kAbstractBit = 3,
+    kStateTagBit = 4,
     kStateTagSize = 2,
   };
   class ConstBit : public BitField<bool, kConstBit, 1> {};
   class InterfaceBit : public BitField<bool, kInterfaceBit, 1> {};
+  class AbstractBit : public BitField<bool, kAbstractBit, 1> {};
   class StateBits : public BitField<RawClass::ClassState,
                                     kStateTagBit, kStateTagSize> {};  // NOLINT