This is a reland of 3c9e924073
Original change's description:
> Revamp patch_sdk.dart mainly to use libraries.json.
>
> This will make it possible to use the script to create a patched sdk
> for dart2js and the vm (some followup needed), which will be useful
> to migrate their libraries to nnbd.
>
> In the future, we should be able to leverage this new logic to
> write tests that ensure our sdk is warning-free (there are > 80 real
> warnings today even on the non-nnbd sdk)
>
> In detail, this CL includes:
>
> * loading the libraries data from libraries.json, this removes the need for a
> hand-crafted libraries.dart that replicates the data in libraries.json.
>
> * auto-generates a libraries.dart in the patched sdk that can be used to run
> dartanalyzer or to compile an sdk with dartdevc-legacy.
>
> * script simplifications in patch_sdk.dart
> * removed tracking of modification stamps. This didn't appeared to be
> used by the ninja rules. I believe it was used to reduce local
> iteration cycles when we were first developing the sdk itself.
>
> * use Uri directly instead of package:path - this simplifies things,
> especially since the libraries_specification is Uri-based as well.
>
> * switch to use package:args for parsing options
>
> * added an option to merge part files in a single file. I've noticed that the
> analyzer provides some false positive errors in part files, but once merged
> they are no longer shown. In particular, we are clearly hitting limitations of
> dartanalyzer on the patched sdk. At this time the old patch-script generated
> an sdk with 1500 warnings, this new script reduces it to 500 (not sure why),
> with merging of part files it goes down to 80.
>
>
> Change-Id: I6bbaf92ef4554f00c8bf6b38d19bf79d44fb3e94
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125780
> Reviewed-by: Nicholas Shahan <nshahan@google.com>
> Commit-Queue: Sigmund Cherem <sigmund@google.com>
Change-Id: Ifcb9dbef36452e41e8e96e086761d76b5a3d988e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125998
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Auto-Submit: Sigmund Cherem <sigmund@google.com>
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