add constructor properties, fixes #60

spec.html

@@ -237,6 +237,11 @@ <h1>AsyncIterator.from ( _O_ )</h1>
     <emu-clause id="sec-iteratorprototype-object">
       <h1>The <dfn>%Iterator.prototype%</dfn> Object</h1>
 
+      <emu-clause id="sec-iteratorprototype-constructor">
+        <h1>%Iterator.prototype%.constructor</h1>
+        <p>The initial value of %Iterator.prototype%.constructor is %Iterator%.</p>
+      </emu-clause>
+
       <emu-clause id="sec-iteratorprototype-map">
         <h1>%Iterator.prototype%.map ( _mapper_ )</h1>
         <p>%Iterator.prototype%.map is a built-in generator function which, when called, performs the following prelude steps:</p>
@@ -479,6 +484,11 @@ <h1>%Iterator.prototype% [ @@toStringTag ]</h1>
     <emu-clause id="sec-asynciteratorprototype">
       <h1>The <dfn>%AsyncIterator.prototype%</dfn> Object</h1>
 
+      <emu-clause id="sec-asynciteratorprototype-constructor">
+        <h1>%AsyncIterator.prototype%.constructor</h1>
+        <p>The initial value of %AsyncIterator.prototype%.constructor is %AsyncIterator%.</p>
+      </emu-clause>
+
       <emu-clause id="sec-asynciteratorprototype-map">
         <h1>%AsyncIterator.prototype%.map ( _mapper_ )</h1>
         <p>%AsyncIterator.prototype%.map is a built-in async generator function which, when called, performs the following prelude steps:</p>