Drop InvalidGenerationSourceError

[natebosch]

This class does not follow best practices, and I think it's outlived it's usefulness. We'd also like to shift the concept of a build failure into the core build system instead of source_gen. The concept of writing "error" output to the generated file is also outdated. I don't think any of our build systems treat the file as having any useful content if it is a failure.

dart-lang/build#981

cc @davidmorgan - this class is used in built_value_generator. I think most of that error handling can be remove entirely. It should be fine to let the exception bubble up.

cc @jakemac53 @leonsenft

[davidmorgan]

Thanks, filed an issue for built_value. Any timeline for this? I guess built_value will need updating before we can roll the updated source_gen to google3.

[natebosch]

We'll hopefully have a migration plan or at least a design for a replacement before dropping it here so no rush for now.

[kevmoo]

FYI a3e3c9a does away w/ the idea of including error messages in source code

[natebosch]

I think we turned out to expand usage of InvalidGenerationSource, and it does seem to be useful enough to keep.