[1.6.1] Missing Reflect in Internet Explorer 11 when app includes a class that extends a built-in class

[y-ich]

Accessing an app by IE11 causes '"Reflect" is not defined' error in modules.js.

To reproduce it,

1. meteor create reproduction.

2. cd reproduction

3. add the following code into client/main.js.

   class DummyError extends Error {}

4. invoke meteor

5. access the page by IE11.

Doesn't Babel 7 support IE11?

[abernix]

This seems related to changes in the way built-ins (e.g. Date, Uint16Array, Error, ...) are able to be extended in Babel 7 (surfacing in babel/babel#7020).

In Babel's wrapNativeSuper, the use of typeof Reflect is transpiled (when published) to use Babel's own _typeof helper function, which IE11 takes issue with (throws) as Reflect is undeclared. It ends up looking like this when shipped to the client:

var _construct = _typeof(Reflect) === "object" && _Reflect$construct || function _construct(Parent, args, Class) {
  var Constructor,
      a = [null];
  a.push.apply(a, args);
  Constructor = Parent.bind.apply(Parent, a);
  return _sPO(new Constructor(), Class.prototype);
};

A native typeof Reflect in IE11 does return "undefined", as expected, but the problem arises specifically with the use of _typeof(Reflect). Additionally, if the line above is changed to the following, everything seems to work okay in IE11:

var _construct = Reflect && _typeof(Reflect) === "object" && _Reflect$construct || function _construct(Parent, args, Class) {

I suppose Meteor could fix this by shipping Reflect polyfills in ecmascript-runtime-client, but this does seem like a bug in Babel and seeing as IE11 is one of two so-called "modern browsers" which doesn't natively support Reflect, it seems like a waste to ship that polyfill if we can avoid it.

This is my take, however @benjamn may know the cleanest way to fix this.

[benjamn]

I believe we can make this work with a patch update to ecmascript-runtime-client by adding just the Reflect.construct polyfill (which is a small subset of the whole Reflect polyfill). Then in Meteor 1.6.2 we can restrict that polyfill to legacy browsers (#9439), since modern browsers (as we're defining them—anything with native async/await) should all have native support for Reflect.construct.