add support for global transformers in code_transformers

[sigmundch]

Implement a generic transformer that:

• can only run in the entrypoint package
• allows specifying modifications to files in any package, but that are only visible to the compilation of a single entrypoint.
• does a functional copy of the import graph, so that all these modifications are supported.

[sigmundch]

Marked this as blocking #19168.

[sigmundch]

Marked this as blocking #15080.

[sigmundch]

Added this to the 1.6 milestone.
Removed Priority-Unassigned label.
Added Priority-High label.

[sigmundch]

Removed this from the 1.6 milestone.
Added Polymer-P-1 label.

[sigmundch]

Removed Polymer-P-1 label.
Added Polymer-Milestone-Next label.

[sigmundch]

Added PolymerMilestone-Next label.

[sigmundch]

Removed Polymer-Milestone-Next label.

[kasperl]

Added this to the 1.6 milestone.

[sigmundch]

Removed this from the 1.6 milestone.

[sigmundch]

Marked this as blocking #19521.

[sigmundch]

Marked this as blocking #20267.

[kasperl]

Added this to the 1.6 milestone.

[sigmundch]

Removed this from the 1.6 milestone.

[sigmundch]

Marked this as blocking #20691.

[justinfagnani]

+Bob and Natalie

Shouldn't this be a basic feature of pub and barback rather than be built as a separate transformer system on top of it?

I need a global transformer for js-interop, and we already have ones that workaround pub's single-package limitation for Polymer, Angular, and any transformer that uses code_transformers.

---

cc @nex3.
cc @munificent.