[2.0] Chaining promises hangs browser (100% CPU)

[Tarpsvo]

Your system information

• VelocityJS version: 2.0.4
• Browser: Chrome 66.0.3359.181
• Operating System: Reproduced on Windows 10 and MacOS 10.13.2

Please describe your issue in as much detail as possible:

Chaining multiple velocity.js promises hangs the Chrome tab and requires the tab to be force killed through task manager, as it no longer reacts to regular exit codes. The processor usage for the tab's process is stuck at 100%.

Steps for reproducing this issue (code):

Minimal code example, using 2.0.4 from CDN: https://jsfiddle.net/Tarps/6kjLrujx/

Pressing "Try animation" button freezes the Chrome browser. Be ready to force quit the tab using Chrome's task manager.

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This is essentially a continuation of #868

[Rycochet]

This is a seriously annoying one. I've cleaned up code slightly, but the problem is that Velocity returns a Promise, which is attached to an object that is then used to resolve the promise, and it seems to get stuck in an infinite loop.

I'm stuck on how to fix it right now, but have an inkling of an idea in returning a different promise to the one that is resolved, and then resolving the one that's returned while having the first .then actually override whatever it outputs with the true output... It's going around in circles and not quite joining the dots - so any help would be appreciated.

I do feel this almost qualifies as a slight break in the spec itself - as there's nothing in it about trying to resolve a promise with itself, and I feel that's overlooked (so undefined behaviour in this case causes a lockup without any major memory issues that I can see).