handle dartdoc references to non-imported symbols (was: dartdoc-specific imports) #1153
Description
Sometimes, a library, like the Flutter rendering library, wants to refer to another library, like the Flutter widgets library, in the documentation, but doesn't want to introduce a dependency in the code. In this case, for example, we definitely do not want any sort of runtime relationship from the rendering library to the widgets library.
We can't use import for this because it risks code accidentally depending through the import.
One possible solution would be to have something like /// @import which would cause the dartdoc code to import it. Another would be for the dartdoc tool to be able to receive a list of imports to imply at the top of every file. I don't really mind how we solve this.