[analyzer] Find TODOs mainly without regex

The RegEx engine in the VM was updated in
e443b89f238cf97b7d33eb54ca34fc19ef44f52f which caused the analyzer
analyzing the CFE to use ~150 mio instructions more.

Part of this was an increased cost in TODO processing which relied on
regex.
The regex was long, complicated (and buggy) and the method scraping todo
comments (including children) when analyzing the CFE cost ~265 mio
instructions (up from ~191 mio instructions before the regex engine update) (numbers from `valgrind --tool=callgrind`).

This CL:
* Adds a test that didn't pass before.
* Removes the usage of the complicated regex with "normal code" that -
  while longer - is likely easier to debug in practice.
* Reduces the cost of scraping to ~92 mio (numbers from
  `valgrind --tool=callgrind`).

`perf stat` on this change with GC disabled goes from

```
    40,938,677,452      instructions:u
    40,935,421,892      instructions:u
    40,935,153,915      instructions:u
```

to

```
    40,756,218,191      instructions:u
    40,756,538,467      instructions:u
    40,756,528,402      instructions:u
```

A ~179 mio savings.

Change-Id: Id02c42d8be2ddbd24b96e33cc89320342efe7f4b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/486800
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>