Consider configuring the builder to run on test/ directory only by default

[evanweible-wf]

More details in dart-lang/build#3238, but currently the mockito builder in this package specifies .dart as the build extension for its inputs and doesn't configure the generate_for option at all. As a result, the builder runs on every dart file in a project, which is significant because the builder is implemented in such a way that it fully resolves every Dart library. For large projects, this is an extremely expensive default behavior, especially considering that this builder was added to this library in a minor release and is auto-applied to any project that already depended on mockito and used build_runner.

Given the nature of this library, it might make more sense for the default configuration to be that it only runs on files in the test/ directory. Running the builder on files in other directories would still be possible, it would just require explicit configuration (which seems justifiable).

A change like this would be breaking, but I think it's worth considering.

[natebosch]

+1 on adding a default generate_for: test/** since thats what most folks will want.

Slight possibility this would break someone, but they could override generate_for to get unstuck.

[cc-nogueira]

+1 on this!

My flutter project has an examples folder and mockito builder was trying to generate mocks in all examples test folders.
This generation spent unnecessary time and gave false errors for types not found.

I have added the following build.yaml to my flutter root (first time I customize flutter builders). Seems to work.

targets:
  $default:
    builders:
      mockito|mockBuilder:
        enabled: true
        generate_for: ["test/**"]