Add failing test demonstrating a ReactPerf warning

[nathanmarks]

Related issue: #6949

When using ReactPerf from a component, I'm getting results back from Perf.getWasted(measurements) but I'm seeing this warning crop up too:

Warning: There is an internal error in the React performance measurement code. We did not expect componentDidMount timer to stop while no timer is still in progress for another instance. Please report this as a bug in React.

cc @gaearon

I simplified the component that I shared in the issue for this failing test. The use case that led me to stumble into this is using a react component as a test fixture to benchmark re-render performance programatically.

The simplified test component cycles through a single state change and resolves in componentDidUpdate before checking the console error output. The warning is actually firing in the componentDidMount hook as soon as ReactPerf.start(); is called.

(here's another jsfiddle with the simplified version: https://jsfiddle.net/69z2wepo/47901/)

[ghost]

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[nathanmarks]

Have emailed to get the CLA sorted.

[gaearon]

Thank you so much! I built #7202 on top of it, let’s discuss there.

[gaearon]

Should be fixed in 15.2.1, can you verify?