Convert polymer transformers to be a "global" transformer

[sigmundch]

We want to use a global transformation for several reasons:
 - make some packages easier to use: we can remove the default transformer from smoke, which sometimes surprises people.

 - faster load: no need to transform code that is loaded by transfomers (we might be able to also remove the self-transformer in polymer itself)

 - make it possible to run the compiler lazily and only when needed in pub-serve (skip some of it for dartium)

 - open the option of only using the polymer-transformer in the final app, not on each widget-package. Currently setting up the transformer section is not very intuitive if you have elements in multiple packages.
 

[sigmundch]

Issue #15038 has been merged into this issue.

[sigmundch]

Issue #15080 has been merged into this issue.

[sigmundch]

see both issue #15080 and issue #15038 for examples illustrating the last point.

[kasperl]

Added this to the 1.6 milestone.

[sigmundch]

Removed this from the 1.6 milestone.

[DartBot]

This issue has been moved to dart-archive/polymer-dart#422.