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

src/js/generator.js

Index: src/js/generator.js
diff --git a/src/js/generator.js b/src/js/generator.js
index 6240f7ad3f0aa288f4e9f3fcf73a670a3e32df2f..4d3f3191254df7b01d5a8c5bcdc06a72d9d0c161 100644
--- a/src/js/generator.js
+++ b/src/js/generator.js
@@ -84,9 +84,11 @@ function GeneratorFunctionConstructor(arg1) {  // length == 1
   var global_proxy = %GlobalProxy(GeneratorFunctionConstructor);
   // 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, GeneratorFunction, new.target);
 }
 
 // ----------------------------------------------------------------------------