Avoid computing the URI scanner tables at runtime.

Precompute the table and store it as a String literal.

Fixes #49108.

Bug: https://dartbug.com/49108
Change-Id: I33d3e768ad2013ad2edec3558fcd3da39b3f4f32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247383
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Auto-Submit: Lasse Nielsen <lrn@google.com>

Author: lrhn