LibJS: Implement Reflect.{isExtensible,preventExtensions}()

Libraries/LibJS/Tests/Reflect.isExtensible.js

@@ -0,0 +1,29 @@
+load("test-common.js");
+
+try {
+    assert(Reflect.isExtensible.length === 1);
+
+    [null, undefined, "foo", 123, NaN, Infinity].forEach(value => {
+        assertThrowsError(() => {
+            Reflect.isExtensible(value);
+        }, {
+            error: TypeError,
+            message: "First argument of Reflect.isExtensible() must be an object"
+        });
+    });
+
+    assert(Reflect.isExtensible({}) === true);
+
+    var o = {};
+    o.foo = "foo";
+    assert(o.foo === "foo");
+    assert(Reflect.isExtensible(o) === true);
+    Reflect.preventExtensions(o);
+    o.bar = "bar";
+    assert(o.bar === undefined);
+    assert(Reflect.isExtensible(o) === false);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}