Fix Function.prototype.length

Summary:
Found by "test262/test/built-ins/Function/prototype/length.js",
`Function.prototype.length` should be non-configurable.
Also, its default value should be 0 instead of undefined.

```
Object.getOwnPropertyDescriptor(Function.prototype, "length")
{ value: 0, writable: false, enumerable: false, configurable: true  }
```

Reviewed By: avp

Differential Revision: D28970136

fbshipit-source-id: 8ea2a98e4494ec49bcd84c4a5918d3de31bc7c7f

lib/VM/JSLib/GlobalObject.cpp

@@ -273,6 +273,12 @@ void initGlobalObject(Runtime *runtime, const JSLibFlags &jsLibFlags) {
   DefinePropertyFlags normalDPF =
       DefinePropertyFlags::getNewNonEnumerableFlags();
 
+  // Not enumerable, not writable but configurable.
+  DefinePropertyFlags configurableOnlyPDF =
+      DefinePropertyFlags::getDefaultNewPropertyFlags();
+  configurableOnlyPDF.enumerable = 0;
+  configurableOnlyPDF.writable = 0;
+
   /// Clear the configurable flag.
   DefinePropertyFlags clearConfigurableDPF{};
   clearConfigurableDPF.setConfigurable = 1;
@@ -356,8 +362,8 @@ void initGlobalObject(Runtime *runtime, const JSLibFlags &jsLibFlags) {
       Handle<JSObject>::vmcast(&runtime->functionPrototype),
       runtime,
       Predefined::getSymbolID(Predefined::length),
-      clearConfigurableDPF,
-      Runtime::getUndefinedValue()));
+      configurableOnlyPDF,
+      Runtime::getZeroValue()));
 
   // [[ThrowTypeError]].
   auto throwTypeErrorFunction = NativeFunction::create(

test/hermes/function-props.js

@@ -13,7 +13,7 @@ function foo() {}
 print("foo.length/configurable:", Object.getOwnPropertyDescriptor(foo, "length").configurable);
 //CHECK: foo.length/configurable: true
 print("foo.__proto__.length/configurable:", Object.getOwnPropertyDescriptor(foo.__proto__, "length").configurable);
-//CHECK: foo.__proto__.length/configurable: false
+//CHECK: foo.__proto__.length/configurable: true
 print("Function.length/configurable:", Object.getOwnPropertyDescriptor(Function, "length").configurable);
 //CHECK: Function.length/configurable: true
 

test/serialize/function-serialize.js

@@ -37,7 +37,7 @@ function foo() {}
 print("foo.length/configurable:", Object.getOwnPropertyDescriptor(foo, "length").configurable);
 //CHECK: foo.length/configurable: true
 print("foo.__proto__.length/configurable:", Object.getOwnPropertyDescriptor(foo.__proto__, "length").configurable);
-//CHECK: foo.__proto__.length/configurable: false
+//CHECK: foo.__proto__.length/configurable: true
 print("Function.length/configurable:", Object.getOwnPropertyDescriptor(Function, "length").configurable);
 //CHECK: Function.length/configurable: true
 

test/serialize/function-usercode.js

@@ -34,7 +34,7 @@ serializeVM(function() {
   print("foo.length/configurable:", Object.getOwnPropertyDescriptor(foo, "length").configurable);
   //CHECK: foo.length/configurable: true
   print("foo.__proto__.length/configurable:", Object.getOwnPropertyDescriptor(foo.__proto__, "length").configurable);
-  //CHECK: foo.__proto__.length/configurable: false
+  //CHECK: foo.__proto__.length/configurable: true
   print("Function.length/configurable:", Object.getOwnPropertyDescriptor(Function, "length").configurable);
   //CHECK: Function.length/configurable: true
 

utils/testsuite/testsuite_skiplist.py

@@ -838,8 +838,6 @@ SKIP_LIST = [
     "test262/test/built-ins/Function/prototype/restricted-property-arguments.js",
     "test262/test/built-ins/Function/prototype/restricted-property-caller.js",
     "test262/test/built-ins/Function/prototype/bind/BoundFunction_restricted-properties.js",
-    # TODO(T90542855) Function.prototype.length should be configurable
-    "test262/test/built-ins/Function/prototype/length.js",
     # Object() changes
     "test262/test/built-ins/Object/prototype/toLocaleString/primitive_this_value.js",
     "test262/test/built-ins/Object/prototype/toLocaleString/primitive_this_value_getter.js",