deps/v8/src/js/prologue.js
function SetUpLockedPrototype(
    constructor, fields, methods) {
  %CheckIsBootstrapping();
  var prototype = constructor.prototype;
  // Install functions first, because this function is used to initialize
  // PropertyDescriptor itself.
  var property_count = (methods.length >> 1) + (fields ? fields.length : 0);
  if (property_count >= 4) {
    %OptimizeObjectForAddingMultipleProperties(prototype, property_count);
  }
  if (fields) {
    for (var i = 0; i < fields.length; i++) {
      %AddNamedProperty(prototype, fields[i],
                        UNDEFINED, DONT_ENUM | DONT_DELETE);
    }
  }
  for (var i = 0; i < methods.length; i += 2) {
    var key = methods[i];
    var f = methods[i + 1];
    %AddNamedProperty(prototype, key, f, DONT_ENUM | DONT_DELETE | READ_ONLY);
    %SetNativeFlag(f);
  }
  %InternalSetPrototype(prototype, null);
  %ToFastProperties(prototype);
}
function SetUpLockedPrototype(
    constructor, fields, methods) {
  %CheckIsBootstrapping();
  var prototype = constructor.prototype;
  // Install functions first, because this function is used to initialize
  // PropertyDescriptor itself.
  var property_count = (methods.length >> 1) + (fields ? fields.length : 0);
  if (property_count >= 4) {
    %OptimizeObjectForAddingMultipleProperties(prototype, property_count);
  }
  if (fields) {
    for (var i = 0; i < fields.length; i++) {
      %AddNamedProperty(prototype, fields[i],
                        UNDEFINED, DONT_ENUM | DONT_DELETE);
    }
  }
  for (var i = 0; i < methods.length; i += 2) {
    var key = methods[i];
    var f = methods[i + 1];
    %AddNamedProperty(prototype, key, f, DONT_ENUM | DONT_DELETE | READ_ONLY);
    %SetNativeFlag(f);
  }
  %InternalSetPrototype(prototype, null);
  %ToFastProperties(prototype);
}
Prevents changes to the prototype of a built-in function. The "prototype" property of the function object is made non-configurable, and the prototype object is made non-extensible. The latter prevents changing the __proto__ property.