[infra] Shard dart2js modular tests

* Shard dart2js modular tests (and decrease the shards of the
  unit tests)
* Remove folders we look for dart files in --- the remove folders
  doesn't contain any anyway and would have to be copied if keeping
  these lines.
* Fix sharding, previously trying to shard in 2 shards would only allow
  you to run ~50% of the tests:

```
$ out/ReleaseX64/dart-sdk/bin/dart pkg/compiler/tool/modular_test_suite.dart -nweb-unittest-asserts-linux --verbose --use-sdk --shards=2 --shard=0
Error: shard should be between 0 and 1, but got 0

$ out/ReleaseX64/dart-sdk/bin/dart pkg/compiler/tool/modular_test_suite.dart -nweb-unittest-asserts-linux --verbose --use-sdk --shards=2 --shard=2
Error: shard should be between 0 and 1, but got 2
```

  This has been corrected to allow from `1..n` for `n` shards to fit
  with what the testing system sends when specifying `shards` in
  `tools/bots/test_matrix.json`.

For previous try runs I extracted this:

Build 60615:
Shard #1: --- Total time: 04:27 ---
Shard #2: --- Total time: 05:19 ---
Shard #3: --- Total time: 04:23 ---
Shard #4: --- Total time: 10:29 ---
 => A total of 24:35 --- combined finish of 10:29
Modular tests: 32:19
Total bot runtime: 35:48

Build 60614:
Shard #1: --- Total time: 03:57 ---
Shard #2: --- Total time: 05:18 ---
Shard #3: --- Total time: 05:21 ---
Shard #4: --- Total time: 05:34 ---
 => A total of 20:10 --- combined finish of 5:34
Modular tests: 29:41 secs
Total bot runtime: 35:55

Build: 60613
Shard #1: --- Total time: 03:56 ---
Shard #2: --- Total time: 05:15 ---
Shard #3: --- Total time: 04:32 ---
Shard #4: --- Total time: 05:34 ---
 => A total of 19:17 --- combined finish of 5:34
Modular tests: 33:39
Total bot runtime: 38:51

With the new sharding I'd estimate that the unit tests and modular tests
would have finished in less than 13 minutes, making the bots finish in
~17 minutes, ~20 minutes and ~19 minutes instead.
The try-run with this ran in 17:06

Possibly a follow-up could do more stuff on the "main bot".

Change-Id: Ie5c96206deb9c0c6db3385bbca04ae6f4eab4c3a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448381
Reviewed-by: Nate Biggs <natebiggs@google.com>
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>