[analyzer] Don't utf8-encode needlessly in ApiSignature.addString

Comparing a before and an after aot-compiled analysis server,
running each 25 times, via `pkg/front_end/tool/benchmarker.dart`:

```
out/ReleaseX64/dart pkg/front_end/tool/benchmarker.dart \
  --iterations=25 \
  --snapshot=pkg/analysis_server/bin/server.aot.2 \
  --snapshot=pkg/analysis_server/bin/server.aot.3 \
  --arguments="--disable-file-byte-store" \
  --arguments="--train-using" \
  --arguments="pkg/front_end/lib/"
```

(where `server.aot.2` is an aot-compile of the parent CL and
`server.aot.3` is an aot-compile with this CL) gets this result:

```
msec task-clock:u: -1.5762% +/- 0.7803% (-131.97 +/- 65.33) (8372.91 -> 8240.93)
page-faults:u: 0.7490% +/- 0.1986% (1161.20 +/- 307.82) (155028.84 -> 156190.04)
cycles:u: -1.6791% +/- 0.8074% (-586734139.48 +/- 282150958.07) (34944191636.32 -> 34357457496.84)
instructions:u: -2.0051% +/- 0.0614% (-926171189.36 +/- 28378206.66) (46190016825.56 -> 45263845636.20)
seconds time elapsed: -1.5786% +/- 0.7789% (-0.13 +/- 0.07) (8.39 -> 8.26)
seconds user: -1.5335% +/- 0.7902% (-0.12 +/- 0.06) (8.02 -> 7.89)
Scavenge(   new space) goes from 134 to 131
MarkSweep(   old space) goes from 1 to 0
Evacuate(store buffer) goes from 1 to 0
Notice combined GC time goes from 2766 ms to 2664 ms (notice only 1 run each).
```

This seems quite high and is more than I've gotten before rebases etc, so I'm guessing this has been lucky with GCs or something.

Running it on `pkg/analyzer/lib/` instead (only 5 times) I get:

```
msec task-clock:u: -2.1880% +/- 1.2579% (-227.09 +/- 130.55) (10378.74 -> 10151.65)
cycles:u: -2.2829% +/- 1.3197% (-988942892.00 +/- 571677673.33) (43320229255.60 -> 42331286363.60)
instructions:u: -1.9075% +/- 0.5803% (-1080089151.00 +/- 328577465.65) (56624410628.20 -> 55544321477.20)
seconds time elapsed: -2.1807% +/- 1.2533% (-0.23 +/- 0.13) (10.39 -> 10.17)
seconds user: -1.9061% +/- 1.4734% (-0.19 +/- 0.15) (9.94 -> 9.75)
Scavenge(   new space) goes from 168 to 164
MarkSweep(   promotion) goes from 17 to 15
MarkSweep(   old space) goes from 4 to 2
Notice combined GC time goes from 3228 ms to 3723 ms (notice only 1 run each).
```

which looks ~similar though so I don't know.

Change-Id: I21c9dd6fd5e122f65ec596551af8a2ec464d3920
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428320
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>