[analyzer] Receive stdin in an isolate to increase responsiveness

Before this CL, typing in a big file in IntelliJ on Windows could lead
to longer and longer latency (read: taking longer and longer from you're
finished writing and the code is error free until the editor doesn't
report an error --- I've reached 40 seconds for a simple print statement
in my testing).

This is caused by a number of things; this CL fixes the issue by
receiving data from stdin in an isolate.

Background:
* The IntelliJ plugin sends the whole file on every change in a file.
  Files can be big though, and e.g. `ast.dart` in the kernel package is
  almost 500 kb.
* The IntelliJ plugin sends the data in chunks of size ~8192 bytes.
* On Windows (but not Linux) it is also received in chunks of that size.
* Darts `LineSplitter` is currently quadratic in behavior
  (https://github.com/dart-lang/sdk/issues/51167) (to be fixed with
  https://dart-review.googlesource.com/c/sdk/+/280100)

Even with the fix of `LineSplitter`, when editing `ast.dart` in IntelliJ
on Windows, (as long as one is typing fast enough) we'll create a longer
and longer queue of messages (with more and more latency).
In testing I reached ~40 seconds of latency, i.e. after I was done
typing and there were no more errors it took ~40 seconds before the
Analyzer and the editor agreed.

I think what's happening is something like this:
* You type a character in IntelliJ in Windows.
* IntelliJ sends two requests: Updated content (the whole file, i.e.
  lots of data) and "give me completions".
* You type another character (I seem to type one character every ~150 ms
  when typing) and it sends two new requests.
* When receiving is slow it processes everything. So it updates the file
  and marks the file for being analyzed. Then it processes the
  completion requests which "forces" the file to be analyzed. This takes
  more than 150 ms, so it gets behind. More and more so for each
  character typed.

Receiving in an isolate means that once the main thread receives
(now via a `ReceivePort`) it receives more data and can now skip some of
the completion requests meaning that it doesn't analyze the file for the
"intermittent" updates.

Change-Id: I43507412abfddcfb903d90d86226172b291bdb47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280321
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>