Pub can fail to load transformers necessary for local development

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#23084

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If a package "foo" uses a transformer from package "bar" and restricts that transformer to only run on files in "test", "bar"'s transformer won't be loaded at all. This is due to a flaw in Pub's logic for determining necessary transformers: because "bar" only runs on non-public file and isn't from the root package, pub assumes that it's not necessary. Instead, pub should check whether it's used in the root package.

This is a serious issue for the test runner: it needs to expose a transformer for its users to use, but that transformer will only be run on tests.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Fixed by dart-lang/sdk@fb830b7.

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Added Fixed label.