unhelpful error message: " A transformer imported unknown package"

[skybrian]

This error message from dependency_computer.dart isn't very helpful, because names neither the transformer nor the dart package where the problem occurred. I can hack the source code to figure this out, but we should have a better error message.

A transformer imported unknown package "protobuf" (in "package:protobuf/protobuf.dart").
FINE: Exception type: ApplicationException
FINE: package:pub/src/utils.dart 935 fail
| package:pub/src/barback/dependency_computer.dart 134 DependencyComputer._transformersNeededByPackageUri
| package:pub/src/barback/dependency_computer.dart 358 _PackageDependencyComputer.transformersNeededByLibrary.
| dart:core Iterable.fold
| package:pub/src/utils.dart 722 unionAll
| package:pub/src/barback/dependency_computer.dart 356 _PackageDependencyComputer.tr