Support Promise execution in the sandbox.

[kevinswiber]

Promise.prototype.then arguments are executed in a microtask queue,
pushed to the end of the current event loop tick. The async
lifetime tracking in the sandbox did not factor this into
consideration. Certain completion functions needed to be deferred
with setImmediate(), which will add to the task queue and
execute after the current microtask queue drains.

Calls to global timers also needed to be wrapped in a scoped context.
Due to the setImmediate() delay, the bridge may disconnect when the
context is disposed before the completion timer fires. Disconnecting
the bridge removes all globals. This prevents reference errors from
happening on the final tick.

Original behavior of execution.return.async remains. It will only
be set to true if there's a pending timer that hasn't been
previously cleared.

A code example that will show the current issue of early exit:

const foo = () => new Promise((resolve) => {
  console.log('foo');
  setTimeout(resolve, 200);
});

const bar = () => new Promise((resolve) => {
  console.log('bar');
  resolve();
});

const baz = () => new Promise((resolve) => {
  console.log('baz');
  resolve();
});

Promise.resolve()
  .then(() => foo())
  .then(() => bar())
  .then(() => baz());

The issue occurs when async tracking happens within the microtask queue. Without these changes, the sandbox believes all async functions are done and exits. Using setImmediate in a couple of places defers that check to the beginning of the next tick, after the current tick's microtask queue is drained.

[kevinswiber]

Note that this PR does not address unhandled Promise rejections. That will need to be a separate PR to the https://github.com/postmanlabs/uvm repo.

[codenirvana]

This looks like a duplicate check from line 167.

[kevinswiber]

@codenirvana The first check is "only queue this if pending is currently at 0." The second check is "only actually run this if pending is still at 0." This is the one we really need. The first check just prevents needless queueing.

[codecov]

Codecov Report

Merging #872 (f019c7a) into develop (8fae465) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #872   +/-   ##
========================================
  Coverage    60.71%   60.71%           
========================================
  Files           12       12           
  Lines          560      560           
  Branches       135      135           
========================================
  Hits           340      340           
  Misses         220      220           

Flag	Coverage Δ
unit	60.71% <100.00%> (ø)

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Impacted Files	Coverage Δ
lib/sandbox/timers.js	74.50% <100.00%> (ø)

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