Normative: Set this appropriately for class extends null

Previously, the [[ConstructorKind]] slot was set to "derived"
which would not allow accessing this in the constructor. We now
set [[ConstructorKind]] to "base" so [[Construct]] allocates and
binds this in the function body (to Object.create(C.prototype),
which will be Object.create(null) as per CDI).

Fixes #543.

spec.html

@@ -19028,7 +19028,7 @@ <h1>Runtime Semantics: ClassDefinitionEvaluation</h1>
         1. Let _constructorInfo_ be the result of performing DefineMethod for _constructor_ with arguments _proto_ and _constructorParent_ as the optional _functionPrototype_ argument.
         1. Assert: _constructorInfo_ is not an abrupt completion.
         1. Let _F_ be _constructorInfo_.[[Closure]].
-        1. If |ClassHeritage_opt| is present, set _F_'s [[ConstructorKind]] internal slot to `"derived"`.
+        1. If |ClassHeritage_opt| is present and _protoParent_ is not *null*, then set _F_'s [[ConstructorKind]] internal slot to `"derived"`.
         1. Perform MakeConstructor(_F_, *false*, _proto_).
         1. Perform MakeClassConstructor(_F_).
         1. Perform CreateMethodProperty(_proto_, `"constructor"`, _F_).