implement configuration specific imports

[jmesserly]

See SDK bug: dart-lang/sdk#24581

Guessing this should be pretty easy, mostly plumbing through the right configuration to Analyzer.

CC @alan-knight

[anders-sandholm]

When do you expect to see this land?

Would be great to get this landed. Look's like this might be the last bit missing in enabling a re-write of package http to use config-specific imports instead of mirrors.

[vsmenon]

This issue was moved to dart-lang/sdk#27302