[ddc, dart2js] readd dart:mirrors

Technically almost no user of ddc should be able to reach this compile-time error if they are using webdev (it appears a couple users do invoke it directly, though).  Project sass demonstrated that users of dart2js can notice this breaking change.

Given that we are bundling many breaking changes for null-safety on the next release, it seems like making imports to dart:mirrors a compile-time error is not strictly necessary at this time.

One idea is to change this to a compile-time error in our tools when users opt-in their libraries or when they enable strong null-safety mode.

Change-Id: I7f12657eba508e2d071dda60087a21475a303d9f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139494
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>