This is a reland of d0f55d0e42
Original change's description:
> [dart2js] Add support for bundling fragments.
>
> Also changes the default to bundling rather than interleaving fragments.
>
> Change-Id: Id79d03a8a8b5be7465b8535f6c9c47dfad120c9c
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/191484
> Commit-Queue: Joshua Litt <joshualitt@google.com>
> Reviewed-by: Stephen Adams <sra@google.com>
> Reviewed-by: Sigmund Cherem <sigmund@google.com>
Change-Id: I0347ddb6dd93eb57f0abc259fc477ec3a9d7231b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/194323
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Joshua Litt <joshualitt@google.com>
