provide option to change relative URL to absolute URL in polymer build

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#20691

---

Provide an option to allow changing relative package/ URLs (derived from HTML imports, CSS, and images) to absolute URLs after invoking pub-build.

Because this might require changing also libraries that are included transitively, this is blocked by first being able to do a global transformation.

[DartBot]

This comment was originally written by davenoti...@gmail.com

---

A related discussion: https://groups.google.com/a/dartlang.org/d/msg/web/szLsH84BpQo/e1xqspqZbBsJ