Move Null class out of the VM isolate.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 6e45f028dd6b18fcb27ca0e28b9f483cf84f88fe..ed73e6b290694cae026078ac4f7ed8527a6bc1fa 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -410,7 +410,6 @@ class Object {
   }
 
   static RawClass* class_class() { return class_class_; }
-  static RawClass* null_class() { return null_class_; }
   static RawClass* dynamic_class() { return dynamic_class_; }
   static RawClass* void_class() { return void_class_; }
   static RawClass* unresolved_class_class() { return unresolved_class_class_; }
@@ -558,7 +557,6 @@ class Object {
   static RawObject* null_;
 
   static RawClass* class_class_;  // Class of the Class vm object.
-  static RawClass* null_class_;  // Class of the null object.
   static RawClass* dynamic_class_;  // Class of the 'dynamic' type.
   static RawClass* void_class_;  // Class of the 'void' type.
   static RawClass* unresolved_class_class_;  // Class of UnresolvedClass.
@@ -3751,9 +3749,7 @@ class AbstractType : public Instance {
   }
 
   // Check if this type represents the 'Null' type.
-  bool IsNullType() const {
-    return HasResolvedTypeClass() && (type_class() == Object::null_class());
-  }
+  bool IsNullType() const;
 
   // Check if this type represents the 'void' type.
   bool IsVoidType() const {