[CFE] Don't serialize platform in suite
[CFE] Don't serialize platform in suite

This takes running the weak suite - on my machine - from

```
[ 07:14 | 100.0% | + 2268 | -    0 ]: weak/variance/unconstrained_inference

             check errors: 0:00:00.517818 ms
stress constant evaluator: 0:00:00.748959 ms
                    print: 0:00:01.357741 ms
                typeCheck: 0:00:09.610927 ms
                   verify: 0:00:17.063594 ms
       match expectations: 0:00:17.186629 ms
                  compile: 0:00:56.952218 ms
      transform component: 0:01:23.143726 ms
                      run: 0:02:24.114175 ms
              write .dill: 0:04:07.470467 ms


real    7m19.983s
user    10m43.053s
sys     1m29.653s
```

to

```
[ 03:07 | 100.0% | + 2268 | -    0 ]: weak/variance/unconstrained_inference

             check errors: 0:00:00.521563 ms
stress constant evaluator: 0:00:00.760032 ms
                    print: 0:00:01.358365 ms
              write .dill: 0:00:02.451508 ms
                typeCheck: 0:00:09.750399 ms
                   verify: 0:00:17.081690 ms
       match expectations: 0:00:17.723327 ms
                  compile: 0:00:55.593688 ms
                      run: 0:01:11.893074 ms
      transform component: 0:01:21.368430 ms


real    3m12.619s
user    4m20.531s
sys     0m41.589s
```

Change-Id: I9d235e84d10df06737789e8765f3b55521697e21
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/246961
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>

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