Make a few optimizations

[jensjoha]

This:

• Make 'allFields' a late field instead
• Cache _inheritedElements in allModelElements call
• Make documentedWhere a late field

Measusred on top of (and compared to) #3659 (I couldn't figure out how to actually do a stacked thing on github):

Very small, JIT:

Difference at 95.0% confidence
        -2052.67 +/- 417.162
        -13.7812% +/- 2.80075%
        (Student's t, pooled s = 184.048)

Very small, AOT:

Difference at 95.0% confidence
        -1370.33 +/- 214.258
        -20.2832% +/- 3.17138%
        (Student's t, pooled s = 94.5287)

Flutter, JIT:

Difference at 95.0% confidence
        -5111.33 +/- 2168.41
        -6.57446% +/- 2.78912%
        (Student's t, pooled s = 956.682)

Flutter, AOT:

Difference at 95.0% confidence
        -3224.33 +/- 599.597
        -5.33441% +/- 0.991988%
        (Student's t, pooled s = 264.537)

So it seems better across the board, but comparably less so for a big application (flutter in this case) --- which makes sense because most of the time is spent in the "generator.generate" (name from the "Runtime performance" when running with -v) where these changes is optimizing the "initializePackageGraph" part...

[srawlins]

Thanks these look fine!