feat(reporter): Replace memoizee library with a trivial solution.

[DavidSouther]

The reporter functionality was pulling the entire memoizee library with all its dependencies for a simple memoization solution, and using almost none of the functionality. This seems like not the most critically performant codepath. This PR replaces that with a good-case solution that lowers the dependency footprint.

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[juliemr]

Looks good to me. @zzo for final review.

[zzo]

please fix the (trivial) travis lint failure

[DavidSouther]

please fix the (trivial) travis lint failure

Done.

[maksimr]

@DavidSouther I dont think it's correct fix.
Property sourceMap may be object, see test. Current solution will return incorrect source map in this case.
Solution with weakMap allow cache object and clean cache when file with source map was removed.

\cc @dignifiedquire @zzo

[dignifiedquire]

Agreed, there is a reason a proper cache was used here instead of this simple solution. Also the weakMap solution should be much more memory efficient than this. Unless there is a good reason please revert this.

[dignifiedquire]

@DavidSouther I really don't mind a couple of files of code, but the question is the actual memory and performance footprint lowered by this? If you are concerned with file size you can implement a version using a weakMap instead, but please not using a simple object.

[zzo]

I believe the concern here is the dependency tree is large for this one module - I'll revert until we can find a better solution

[dignifiedquire]

@zzo which I appreciate, but it shouldn't come at the cost of actual runtime behaviour and performance.

[zzo]

agreed thanks for weighing in

[DavidSouther]

@zzo @dignifiedquire @maksimr Correct, I wanted this to remove the rather large memoizee dependency that's used in this one internal place.

I totally misread the tests, and thought only the string sourcemap contents were being used here. Because engines is still 0.10 || 0.12, WeakMap would be inappropriate. This seems like not the most performance critical code - it will be called once per error in testing output. What do you all think about using the string form of the sourcemap? sourceMap.file || sourceMap.contents || (typeof sourceMap == 'string' ? sourceMap : JSON.Stringify(sourceMap).

[dignifiedquire]

We are already including a shim for weakMap, core-js (https://github.com/karma-runner/karma/blob/master/package.json#L233) so I suggest using that.

[DavidSouther]

Ah, perfect! Should I re-open this PR, or start a new PR?

[dignifiedquire]

Let's make it a new PR to have a clean slate (and I think Github gets confused otherwise)