[ddc] Create fork of compiler.dart

The fork (compiler_new.dart) will contain changes to compile libraries
in isolation even within the same module. It will eventually replace
compiler.dart.

Change-Id: I8b1c99895a6618ae6f8e8ed5173886e82ce6364b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/379260
Reviewed-by: Nate Biggs <[email protected]>
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>