define variables in pub build for dart2js

[sigmundch]

This should be a simple change, but it will be extremely useful.

What I'd like is to let codegenerators and runtime libraries to adapt their behavior based on the intended use of the code (distinguishing between dev time and deploy time and what kind of compilation strategy was used (debug/release))

We should be able to do so if we can pass 2 flags to dart2js automatically when calling pub build. I basically would like:

-Ddeployment=true
-Dmode=debug or release

Could we do that :)? pretty please?

[sigmundch]

Removed Type-Defect, Priority-Unassigned labels.
Added Type-Enhancement, Priority-High labels.

[nex3]

Added Duplicate label.
Marked as being merged into #16370.

[DartBot]

This issue has been moved to dart-lang/pub#848.