Implement configuration specific imports

[floitschG]

Implement configuration specific imports [0] behind a flag.
Validate it, and then remove the flag, once the DEP committee has accepted it.

Agreed upon flag (so far): --conditional-directives

The implementation is already underway (checked when committed):

• VM: https://codereview.chromium.org/1380383002/
• dart2js: https://codereview.chromium.org/1388523002/

Still missing: environment variables for the existing libraries:

• VM: issue Set predefined environment variables for existing libraries in the VM #24587
• dart2js: issue Set predefined environment variables for existing libraries in dart2js #24586

Validate implementation by updating pkg/http

• Restructure http/browser_client.dart in http package to avoid dart:mirrors usage (?) http#25

The analysis can be implemented in different phases.

• analyzer: Implement configuration specific imports in the analyzer #24579
  • phase 0
  • phase 1 - no longer planned.
• dart2js (optional): Implement analysis for configuration specific imports in dart2js. #24580 - no longer planned.

Once the DEP committee has accepted:

• remove flag
• Implement in DDC implement configuration specific imports dart-archive/dev_compiler#595
• Validate behavior for summaries and bazel Validate summary generation and bazel model for Config Specific Imports (CSI) #28247

[0] https://github.com/munificent/dep-interface-libraries/blob/master/Proposal.md

[floitschG]

/cc @mit-mit

[bwilkerson]

I now have a CL that will add support in the parser and AST structure for this dep (https://codereview.chromium.org/1406253004/). However, it's a breaking change that will affect many clients of analyzer, so I am hesitant to commit it before we've released 1.13.

[mit-mit]

Now that we are starting 1.14 builds, let's go ahead and land CLs that are ready for this!

[mit-mit]

Looks like the VM changes landed :-). @floitschG, what else remains in 1.14?