introduce web_components pkg for consolidated polyfills

pkg/web_components/pubspec.yaml

Index: pkg/web_components/pubspec.yaml
diff --git a/pkg/web_components/pubspec.yaml b/pkg/web_components/pubspec.yaml
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+name: web_components
+version: 0.9.0

[Siggi Cherem (dart-lang), 2014/02/10 20:49:23]

this doesn't matter, but maybe we want to start at 0.10.0 since all others seem
to be approaching that number :)?

[Jennifer Messerly, 2014/02/10 22:17:11]

yeah, I was thinking 0.9 since it will get to 0.10 before long ... will likely
rev faster than the others. Actually given that line of reasoning, how about
0.3?  gives us the most headroom.

as far as staying in sync, personally I doubt that will happen, I think we need
to treat versions independently.

+author: Polymer.dart Authors <web-ui-dev@dartlang.org>
+homepage: https://www.dartlang.org/polymer-dart/
+description: >
+  Custom Elements let authors define their own elements. Authors associate code
+  with custom tag names, and then use those custom tag names as they would any
+  standard tag. Shadow DOM is designed to provide encapsulation for custom
+  elements, by hiding DOM subtrees under shadow roots.

[Siggi Cherem (dart-lang), 2014/02/10 20:49:23]

not sure whether it helps or makes things more confusing, but should we add a
note that this is not the same as web_components < 0.4?

[Jennifer Messerly, 2014/02/10 22:17:11]

it actually went to 0.2.8...

But yeah, let's not worry about it :). That was like 1.5 yrs ago, I doubt anyone
even remembers.

Besides, it actually is the same package! web_components was our first attempt
at polyfilling CE and Shadow DOM. It is just much more sophisticated now ;)

+environment:
+  sdk: ">=1.2.0-dev.4.0 <2.0.0"