Observed this on Mobile Chrome/33.0.1750.170 using zone.js 0.8.26:

Although that's a pretty outdated browser, I'm still reporting it just in case the issue occurs with other userAgents too.

I tracked the problem down to the following code snippet that patches the then-method of Promise

  function patchThen(Ctor: Function) {
    const proto = Ctor.prototype;

    const prop = ObjectGetOwnPropertyDescriptor(proto, 'then');
    if (prop && (prop.writable === false || !prop.configurable)) {
      // check Ctor.prototype.then propertyDescriptor is writable or not
      // in meteor env, writable is false, we should ignore such case
      return;
    }

    const originalThen = proto.then;
    // Keep a reference to the original method.
    proto[symbolThen] = originalThen;
.
.
.

In case of chrome 33 then is initially not configurable, hence patchThen returns before the original method is patched.

Compared to current chrome:

Imho the reference to originalThen should be kept, even if the properties indicate that it is not writable or not configurable. This was the case prior to the changes with #1041 .