[analyzer] Add benchmark that calls getFixes on an actual error

This CL adds a benchmark for getFixes calls on an actual error case.
The time it takes is nowhere near the times we're seeing in
http://b/407797012 but it's better than nothing.

The benchmark does 5 requests for `getFixes`.
The first one is always much slower because it has to "discover files".

Comparing old dart releases and CLs
https://dart-review.googlesource.com/c/sdk/+/420323 and
https://dart-review.googlesource.com/c/sdk/+/421220:

3.7.2 was quite a bit slower than 3.6.2.
Both CL 420323 and CL 421220 improve things - both separately and combined.
With both we're faster than 3.6.2 for all but the first getFixes call
(which has to "discover files").

Details:

```
Comparing 3.5.4 with 3.6.2
no change.

Comparing 3.6.2 with 3.7.2
Fixes (1): 16.4789% +/- 8.3240% (0.23 +/- 0.12) (1.39 -> 1.62)
Fixes (2): 300.2940% +/- 21.5781% (0.06 +/- 0.00) (0.02 -> 0.08)
Fixes (3): 220.0321% +/- 23.8171% (0.05 +/- 0.01) (0.02 -> 0.07)
Fixes (4): 89.0472% +/- 86.0948% (0.03 +/- 0.03) (0.04 -> 0.07)
Fixes (5): 202.9467% +/- 22.5906% (0.04 +/- 0.00) (0.02 -> 0.07)

Comparing 3.7.2 with HEAD-ish without the two CLs
Fixes (1): 9.2103% +/- 6.7559% (0.15 +/- 0.11) (1.62 -> 1.77)
Fixes (2): -32.2017% +/- 7.2433% (-0.03 +/- 0.01) (0.08 -> 0.06)
Fixes (3): -39.1903% +/- 7.4656% (-0.03 +/- 0.01) (0.07 -> 0.04)
Fixes (4): -41.5019% +/- 8.2911% (-0.03 +/- 0.01) (0.07 -> 0.04)
Fixes (5): -38.7623% +/- 4.4327% (-0.03 +/- 0.00) (0.07 -> 0.04)

Comparing HEAD-ish without the two CLs with CL 420323 (cache) only
Fixes (1): -5.9976% +/- 4.4944% (-0.11 +/- 0.08) (1.77 -> 1.66)
Fixes (4): -39.3839% +/- 13.3201% (-0.02 +/- 0.01) (0.04 -> 0.02)
Fixes (5): -49.0495% +/- 4.2557% (-0.02 +/- 0.00) (0.04 -> 0.02)

Comparing CL 420323 (cache) only with CL 421220 (element2) only
Fixes (2): -63.4332% +/- 42.9828% (-0.02 +/- 0.02) (0.04 -> 0.01)
Fixes (3): -79.6318% +/- 17.5009% (-0.04 +/- 0.01) (0.05 -> 0.01)
Fixes (4): -68.4995% +/- 11.7414% (-0.02 +/- 0.00) (0.02 -> 0.01)
Fixes (5): -54.6697% +/- 11.1116% (-0.01 +/- 0.00) (0.02 -> 0.01)

Comparing CL 421220 (element2) only with both
Fixes (2): -21.3437% +/- 7.8302% (-0.00 +/- 0.00) (0.01 -> 0.01)
Fixes (3): -17.7975% +/- 10.4789% (-0.00 +/- 0.00) (0.01 -> 0.01)
Fixes (5): -30.5353% +/- 20.5827% (-0.00 +/- 0.00) (0.01 -> 0.01)
```

Change-Id: I477c7a73c669099e2d7bc9d64000cfb78ced2ea4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/421601
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>