CDE Polymer Designer implementation

@rginda @devoncarew 

This is a 99% MVP for #2722. The only 2 limitations I'm aware of are:

* The designer element's height in the dialog is hard-coded to 600px: relative sizing has problems which I'm working on.
* Code generation simply overwrites the file without creating an undo point in the editor. I'm working on that too (the way to access the designer might change as the result, as right now one can invoke it on any file, not necessarily loaded into an editor's session).

The core is in cde_polymer_designer.dart. Later on I'm planning to split out the webview-specific stuff into a generic element, possibly picking up and improving the no-longer-maintained https://github.com/rmsmith/webview/.

![screen shot 2014-09-30 at 1 19 27 am](https://cloud.githubusercontent.com/assets/5606182/4455095/907cffc2-487a-11e4-82cb-5f14908a6977.png)

Ah, good old craw days :) I'll try to remember what we did and why, and apply that here.

With regards to `eval()`: as far as I understand, `eval` would execute in the isolated world of the webview -- any JS would for that matter. The only way to push a piece of JS over the isolated world/main world boundary is via this artificial `<script>` tag. That the recipe the webview team pointed me to, so presumably it's the right one :)