Add test for extending a constructor with null .prototype (#806)

...ements/class/constructor-inferred-observable-iteration.js

...ss/class-definition-null-proto-missing-return-override.js

...ments/class/subclass/class-definition-null-proto-super.js

...ements/class/subclass/class-definition-null-proto-this.js

.../statements/class/subclass/class-definition-null-proto.js

...ents/class/subclass/class-definition-parent-proto-null.js