[parser] Change NullValue usage

This gets rid of (what is described in commented code as)
InstantiatedTypeWithArgumentsTest, Subtype1TestCacheLookup, BoolTest.

Running this through the benchmarker like this:

```
out/ReleaseX64/dart-sdk/bin/dart pkg/front_end/tool/benchmarker.dart \
  --snapshot=pkg/analysis_server/bin/server.aot.1 \
  --snapshot=pkg/analysis_server/bin/server.aot.2 \
  --arguments="--disable-file-byte-store" \
  --arguments="--train-using" \
  --arguments="pkg/front_end/lib/"
```

(there `server.aot.1` is without this CL and `server.aot.2` is with)
I get:

```
msec task-clock:u: -0.9614% +/- 0.3557% (-78.23 +/- 28.95) (8136.80 -> 8058.57)
cycles:u: -1.0257% +/- 0.3626% (-348283560.66 +/- 123129577.69) (33955507948.94 -> 33607224388.28)
instructions:u: -1.0595% +/- 0.0662% (-479117043.74 +/- 29935955.93) (45220681554.68 -> 44741564510.94)
seconds time elapsed: -0.9548% +/- 0.3540% (-0.08 +/- 0.03) (8.15 -> 8.07)
seconds user: -1.0649% +/- 0.4031% (-0.08 +/- 0.03) (7.80 -> 7.72)
Scavenge(   new space) goes from 130 to 129
Notice combined GC time goes from 2613 ms to 2608 ms (notice only 1 run each).
```

`StackImpl.pop` as reported by `perf report --no-children` goes from
0.81% to 0.34% (and I'm unsure why the apparent change seen as a whole
is quite a bit bigger).

Change-Id: Ife36da11215e0c8aa706d945f0ee5018ad03ab91
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432340
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>