Replace record keys in maps

[jensjoha]

Something weird is happening - for dart JIT - when using records as keys in maps. See also dart-lang/sdk#54928 where I reported it as a bug.

This PR uses 2 classes instead.
The result - for a very small example - is:

Difference at 95.0% confidence
        -6286.33 +/- 969.339
        -29.6791% +/- 4.57645%
        (Student's t, pooled s = 427.663)

(from ~21 seconds to ~15 seconds)

and trying on flutter the result is

Difference at 95.0% confidence
        -46593 +/- 2604.32
        -37.4728% +/- 2.09454%
        (Student's t, pooled s = 1149)

(from ~124 seconds to ~78 seconds)

There is ~no difference for an AOT compile.

[srawlins]

Thanks for the great analysis, @jensjoha! The gardeners had noticed the dart doc test taking a long time as well, and filed dart-lang/sdk#53278, which I have not looked at in a while. I'll land this change and watch that bot.

[srawlins]

Very straightforward, thanks for the fix!