Introduce a BuiltinsConstructStub that sets up new.target and does a [[call]] per ES6 9.3.2

src/js/v8natives.js

Index: src/js/v8natives.js
diff --git a/src/js/v8natives.js b/src/js/v8natives.js
index 06c315b8906c2df04135c4f9b849095bd69ee64e..21e15427d9a8feaac55b6515f3fb3ae3e3374341 100644
--- a/src/js/v8natives.js
+++ b/src/js/v8natives.js
@@ -1374,11 +1374,11 @@ function NumberConstructor(x) {
   // TODO(bmeurer): Move this to toplevel.
   "use strict";
   var value = %_ArgumentsLength() == 0 ? 0 : TO_NUMBER(x);
-  if (%_IsConstructCall()) {
-    %_SetValueOf(this, value);
-  } else {
-    return value;
-  }
+  if (IS_UNDEFINED(new.target)) return value;
+
+  var result = %NewObject(GlobalNumber, new.target);
+  %_SetValueOf(result, value);
+  return result;
 }