[scanner] Use 'vm:unsafe:no-bounds-checks' and add explicit bounds checks

The (utf8) scanner currently has this thing where you give it a
0-terminated byte-array (i.e. you read the file, then allocate
something that's 1 bigger, copy the data, then give it to the scanner)
to 'avoid bounds checks'.
Dart still has bounds checks though - they're just implicit.

As for the string scanner ut gets a string, then creates a new string
like `string + '\x00'` - so basically the same thing.

This CL uses the 'vm:unsafe:no-bounds-checks' pragma, removing the
implicit bounds checks, adding explicit bounds checks,
saving ~73.6 mio instructions when compiling the CFE in the process:

```
Comparing snapshot #1 with snapshot #2
cycles:u: -0.9983% +/- 0.6563% (-174026333.30 +/- 114410028.98)
instructions:u: -0.3416% +/- 0.0005% (-73659267.00 +/- 108567.20)
branch-misses:u: -4.8952% +/- 2.2612% (-3172939.50 +/- 1465641.18)
```

With the scanner-benchmark with `--bytes` I get this:

```
msec task-clock:u: -1.2251% +/- 0.6355% (-50.64 +/- 26.27)
cycles:u: -1.2376% +/- 0.6385% (-223642830.80 +/- 115393789.68)
instructions:u: -2.8155% +/- 0.0000% (-1153243856.00 +/- 428.11)
seconds time elapsed: -1.2165% +/- 0.6408% (-0.05 +/- 0.03)
seconds user: -1.1539% +/- 0.6495% (-0.05 +/- 0.03)
```

With the scanner-benchmark with `--string` I get this:

```
msec task-clock:u: -7.6439% +/- 0.6628% (-366.08 +/- 31.74)
page-faults:u: -95.0034% +/- 0.0014% (-228023.50 +/- 3.41)
instructions:u: 2.1041% +/- 0.0000% (897941907.60 +/- 2082.79)
branch-misses:u: 3.2994% +/- 1.4675% (3239735.30 +/- 1440940.88)
seconds time elapsed: -7.6595% +/- 0.6610% (-0.37 +/- 0.03)
seconds user: -0.8801% +/- 0.7676% (-0.04 +/- 0.03)
seconds sys: -92.0140% +/- 2.8075% (-0.33 +/- 0.01)
MarkSweep(   old space) goes from 6 to 0
Notice combined GC time goes from 112 ms to 41 ms (notice only 1 run each).
```

Where I'll note that the 'vm:unsafe:no-bounds-checks' pragma doesn't
(yet?) work for `String.codeUnitAt`.
See https://dart-review.googlesource.com/c/sdk/+/384540
(and https://dart-review.googlesource.com/c/sdk/+/385201) for details.
I assume the relatively  big change here is caused by not allocating
a new string with a 0-byte in the end each time.

Note that the read-allocate-copy dance is still performed for the utf8
scanner in this CL as it requires changing all call-sites instead.
It will be done in a follow-up CL where the "end-of-file" int will
likely also be changed to `-1` to (I assume) allow for having the
0-byte in the middle of a file (see also the 10+ year old bug at
https://github.com/dart-lang/sdk/issues/18090)

Note: The pragma (currently?) only has effect in AOT and this change
will (for the utf8 scanner) make the JIT version slower
(probably by the same ~73.6 mio instructions as - at least in AOT -
the implicit check is 6 instructions and the explicit one is 3
instructions). As the pragma doesn't work in the StringScanner anyway
I expect the change to be somewhat equivalent there. Once the
read-allocate-copy dance is also removed from the utf8 scanner I expect
the combined result to be positive all around.

Update: With https://dart-review.googlesource.com/c/sdk/+/385201 landed
I get these changes:

Compiling the CFE:
```
instructions:u: -0.4520% +/- 0.0002% (-98470955.29 +/- 42253.40)
```

Scanner benchmark with `--bytes`:

```
msec task-clock:u: -2.1758% +/- 0.2316% (-92.07 +/- 9.80)
cycles:u: -2.1941% +/- 0.2283% (-405224983.11 +/- 42160655.88)
instructions:u: -3.1049% +/- 0.0000% (-1272360052.95 +/- 706.54)
branch-misses:u: 2.4718% +/- 0.5142% (2371345.23 +/- 493257.76)
seconds time elapsed: -2.1761% +/- 0.2317% (-0.09 +/- 0.01)
seconds user: -2.2071% +/- 0.2308% (-0.09 +/- 0.01)
```

Scanner benchmark with `--string`:

```
msec task-clock:u: -15.0073% +/- 0.2175% (-745.93 +/- 10.81)
page-faults:u: -95.0035% +/- 0.0003% (-228024.25 +/- 0.81)
cycles:u: -7.7986% +/- 0.2329% (-1558985588.99 +/- 46560962.79)
instructions:u: -3.7054% +/- 0.0000% (-1581977447.66 +/- 481.68)
branch-misses:u: -0.6880% +/- 0.5818% (-689453.22 +/- 583101.50)
seconds time elapsed: -15.0198% +/- 0.2170% (-0.75 +/- 0.01)
seconds user: -8.8149% +/- 0.2648% (-0.41 +/- 0.01)
seconds sys: -94.1247% +/- 1.6444% (-0.34 +/- 0.01)
MarkSweep(   old space) goes from 6 to 0
```

Change-Id: I524a21f488da7df5dc9d2cdf40112b84896ad3e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/383324
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

https://dart.googlesource.com/sdk/+/c242d3a7563a5d5c6d0f8a56ad74a51b17f9fac9