Fix test for instantiating a class extending null (#813)

The sameValue assertion should be that the [[Prototype]] is
equal to the class's prototype, not the constructor.

test/language/statements/class/subclass/class-definition-null-proto-missing-return-override.js

@@ -38,4 +38,4 @@ class Foo extends null {
 
 var foo = new Foo();
 
-assert.sameValue(Object.getPrototypeOf(foo), Foo);
+assert.sameValue(Object.getPrototypeOf(foo), Foo.prototype);