Nearly completely implement named properties visibility algorithm

lib/constructs/interface.js

@@ -31,6 +31,57 @@ Interface.prototype.generateNamedPropertiesObject = function () {
     return;
   }
 
+  let overrideBuiltins = false;
+  const unforgeables = new Set();
+  let parent = this.idl;
+  while (parent) {
+    if (utils.getExtAttr(parent.extAttrs, 'Unforgeable')) {
+      unforgeables.add('valueOf').add('toString');
+    }
+    if (utils.getExtAttr(parent.extAttrs, 'OverrideBuiltins')) {
+      overrideBuiltins = true;
+    }
+
+    const members = parent.members.filter((m) =>
+      m.type === 'attribute' && utils.getExtAttr(m.extAttrs, 'Unforgeable')
+    );
+    for (const member of members) {
+      unforgeables.add(member.name);
+    }
+    const parentInterface = this.opts.interfaces[parent.inheritance];
+    parent = parentInterface && parentInterface.idl;
+  }
+
+  this.str += `
+function namedPropertiesIsVisible(P, O) {
+  if (P of ${JSON.stringify(Array.from(unforgeables))}) {
+    return false;
+  }
+  if (!supported) {
+    return false;
+  }`;
+
+  if (overrideBuiltins) {
+    this.str += `
+  return true;`;
+  } else {
+    this.str += `
+  if (Object.getOwnPropertyDescriptor(O, P)) {
+    return false;
+  }
+
+  let prototype = Object.getPrototypeOf(O);
+  while (prototype) {
+    if (prototype.constructor.name.endsWith("PropertiesConstructor") && Object.getOwnPropertyDescriptor(prototype, P)) {
+      return false;
+    }
+    prototype = Object.getPrototypeOf(prototype);
+  }`;
+  }
+  this.str += `
+  return true;
+}`;
+
   this.str += `var ${this.name}PropertiesConstructor = function () {
   throw new TypeError("Illegal constructor");
 }\n`;

lib/transformer.js

@@ -92,7 +92,8 @@ class Transformer {
             obj = new Interface(instruction, {
               implDir: path.resolve(outputDir, file.impl),
               implSuffix: this.options.implSuffix,
-              customTypes
+              customTypes,
+              interfaces
             });
             interfaces[obj.name] = obj;
             break;