[es6] Fix Function and GeneratorFunction built-ins subclassing.

src/js/v8natives.js

Index: src/js/v8natives.js
diff --git a/src/js/v8natives.js b/src/js/v8natives.js
index 3c1e175a9e7f4f1bd8b23678b85c6722ea1d9f9a..d830ba5aca6c5f343ed4ee9e6c4c009e475b643b 100644
--- a/src/js/v8natives.js
+++ b/src/js/v8natives.js
@@ -1797,9 +1797,11 @@ function FunctionConstructor(arg1) {  // length == 1
   var global_proxy = %GlobalProxy(FunctionConstructor);
   // Compile the string in the constructor and not a helper so that errors
   // appear to come from here.
-  var f = %_CallFunction(global_proxy, %CompileString(source, true));
-  %FunctionMarkNameShouldPrintAsAnonymous(f);
-  return f;
+  var func = %_CallFunction(global_proxy, %CompileString(source, true));
+  // Set name-should-print-as-anonymous flag on the ShareFunctionInfo and
+  // ensure that |func| uses correct initial map from |new.target| if
+  // it's available.
+  return %CompleteFunctionConstruction(func, GlobalFunction, new.target);
 }