Add "secret" compiler options to allow dart2js and pub to communicate names of output files to compiler API

[peter-ahe-google]

Problem:

To enable sources maps, you need to append the name of the source map file to the generated output from dart2js (out.js or out.dart by default).

The compiler API (sdk/lib/_internal/compiler/compiler.dart) isn't aware of the these names, so it cannot append these tags. So the names are appended by the dart2js command-line wrapper (sdk/lib/_internal/compiler/implementation/dart2js.dart).

This means that pub (and any other client of the compiler API) has to duplicate this logic.

Possible solutions:

1. My preferred long-term solution is to change the compiler API (issue Compiler API is not aware of URI of output and cannot add sourceMappingURL #15607).
2. Add "secret" options to the compiler API (more options can be easily added as the interface is a list of strings). These secret options are never used from the command line but can inform the compiler API about the location of certain files. This bug report is a suggestion to implement this, as I believe changing the compiler API is a longer-term effort.

[peter-ahe-google]

Marked this as blocking #15563.

[floitschG]

Set owner to @johnniwinther.
Removed Priority-Unassigned label.
Added Priority-High label.

[johnniwinther]

Added Started label.

[johnniwinther]

Fixed by https://codereview.chromium.org/189563004/

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Added Fixed label.