Avoid computing transition tables used for Uri scanning at runtime, instead precompute them and make corelibs use a constant

[mkustermann]

As part of starting an isolate we run bootstrapping code. Part of that will use the Uri class which triggers lazy creation of Uri scanning tables, see sdk/lib/core/uri.dart:

final List<Uint8List> _scannerTables = _createTables();

List<Uint8List> _createTables() {
  // TODO(lrn): Use a precomputed table.
  ... lots of logic here ...
}

=> This is not good for startup latency in general.
=> It is a base cost for any Isolate.spawn(). Making it less appealing to use isolates for short-lived tasks.

The tables can be pre-computed and baked in as a constant into the core libraries, addressing the existing todo in the code.

/cc @lrhn

[lrhn]

I was planning to make the table a single string, but stopped for some reason that I can no longer remember.
(Could be that we were considering a rewrite of Uri for Dart 2.0 anyway, it just didn't make the final list.)

[kevmoo]

Fix reverted in 855e1cd

[lrhn]

Relanded, and appears to have stuck.