[scanner] Specialized scanner recovery for missing end curly brace

*TL;DR*

This improves scanner recovery for a missing `}` in certain situations,
reducing the risk of an in-body change causing a (temporary) outline
change (which in turn could result in the analyzer becoming unresponsive
for "no reason").

*Details*

The behavior of IntelliJ is that when typing `{` it only inserts a
matching end brace `}` when hitting enter.

Imagine you are typing an if: `if (1 + 1 == 2) {`, where you don't hit
enter quickly enough and you trigger a re-analysis at this point.

What happens then is that every method below where you are typing looks
to be local function declarations and thus the outline change. When the
outline change the analyzer has to do a lot of work: everything
(transitively) depending on the file has to be recompiled, and every
strongly connected component is compiled "in one go" where the analyzer
can't respond to queries. So if you have one or more large strongly
connected components depending on the file, or the file itself is part
of such a chain, you will (or at least might) experience that the
analyzer is slow to respond, and it will be extra puzzling because
logically you're just doing an in-body change.

For some code the user might not even naturally hit enter, e.g. `var foo
= {"I'm", "a", "set"};`.

The recovery in the scanner has always been that - upon reaching the end
of the file - it sees that we're missing a `}` and it inserts it at the
end. This CL instead tries to figure out a better place to insert it,
and if successful, will rerun the scanner, instructing it to insert it
at the better place and (hopefully) avoiding a subsequent outline
change.

It does this by looking at the indentation - which is new for recovery -
and under the assumption that the indentation was correct before, will
find the position where the start curly brace was inserted. Note that if
it finds a position it will always be between the start curly brace (the
one missing the end curly brace) and the end of file, and inserting the
missing curly end brace there can't really be "more wrong" than
inserting it at the end (if the new place is not correct it's just
"still wrong").

In the benchmark added we see how quickly we can get completion after
having typed `if (1+1==2) {`, then adding `\n ge\n}` and requesting
completion on the `ge` part, i.e. a simulation of typing

```
if (1+1==2) {
  ge
}
```

and asking for completion at the `ge`.

The change in this CL - on cycles of size 1024 - caused the time to
completion response to come in between ~5 times faster (going from ~10.2
to ~2.1 seconds) to ~18 times faster (going from ~10.3 seconds to ~0.56
seconds):

`CodeType.ImportExportCycle` goes from:

```
+------+-----------+------------+
| Size |  Initial  | Completion |
+------+-----------+------------+
|   16 |  2.019581 |    0.97504 |
|   32 |  3.028976 |   1.031008 |
|   64 |  4.422884 |   1.198383 |
|  128 |  7.612125 |   1.597091 |
|  256 | 12.860864 |   2.906553 |
|  512 | 24.391894 |   5.017093 |
| 1024 | 48.390993 |  10.243085 |
+------+-----------+------------+
```

to

```
+------+-----------+------------+
| Size |  Initial  | Completion |
+------+-----------+------------+
|   16 |  2.107213 |   0.661066 |
|   32 |  3.012952 |    0.70554 |
|   64 |  4.682508 |   0.731176 |
|  128 |  7.508434 |   0.745501 |
|  256 | 13.105477 |   0.852413 |
|  512 | 24.520184 |   1.278403 |
| 1024 | 48.804348 |    2.11903 |
+------+-----------+------------+
```

and `CodeType.ImportExportChain` goes from:

```
+------+-----------+------------+
| Size |  Initial  | Completion |
+------+-----------+------------+
|   16 |  2.059196 |   0.892082 |
|   32 |  3.080717 |    0.93232 |
|   64 |  4.647163 |   1.240303 |
|  128 |  7.377035 |   1.674859 |
|  256 | 12.939432 |   2.705483 |
|  512 | 24.529501 |    5.02689 |
| 1024 | 47.713553 |  10.385469 |
+------+-----------+------------+
```

to

```
+------+-----------+------------+
| Size |  Initial  | Completion |
+------+-----------+------------+
|   16 |  2.020809 |   0.709643 |
|   32 |  3.106856 |   0.648818 |
|   64 |  4.503067 |   0.593152 |
|  128 |   7.45692 |   0.622423 |
|  256 | 13.140592 |   0.606948 |
|  512 | 24.933216 |   0.612687 |
| 1024 | 50.167541 |   0.567544 |
+------+-----------+------------+
```

Change-Id: I8dbefe215162d00a209206ae3db83b2b17505853
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415581
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

https://dart.googlesource.com/sdk/+/5ba59342015d30e6e79f9da30d7f696f7ab818d5