Fix deployed Polymer UI

@terrylucas - widgets/
@dinhviethoa - grind.dart, spark_bootstrap.js, spark_polymer.html, spark_polymer_ui.html

Sorry for another large CL. This one fixes the deployed Polymer UI.

* Do away with spark_bootstrap.js: simply `ln -s *.precompiled.js *.js` -- simpler code, less black magic during deploy and load, faster load.
* Fixed the styling of the deployed Polymer UI:
 * In addition to the existing `@host {...}` CSS blocks in the elements' definitions, added analogous `:host {...}` blocks -- this is a new syntax understood by Chromium, but not yet understood by Dartium (that's why both are temporarily required).
* Fixed data- and event-bindings in the deployed Polymer UI. In particular, fixed spark-overlay, which in turn fixed spark-menu-button not expanding and spark-modal dialogs not closing on a button click:
 * Replaced simple data binding for the `SparkOverlay.opened` attribute with a getter/setter pair, where the setter force-calls `openedChanged()` handler -- simple binding didn't work -- external and internal changes to `opened` didn't trigger `openedChanged()`, unlike in the undeployed code.
 * Correspondingly, for the deployed code, force-set `opened` on the underlying `SparkOverlay` in `SparkMenuButton.toggle()`. Again, simply binding `SparkOverlay.opened` to `SparkMenuButton.opened` via `opened="{{opened}}"` in the HTML didn't work.
 * Additionally added `::content` CSS blocks analogous to the existing `::-webkit-distributed` blocks, which are being phased out. This is not essential for the fix, but complements `:host`-related changes.

The only yet not fixed problem in the deployed code is that the menu button's image is not immediately visible. The first click on it, however, reveals the image. The root of the problem is the same as it was for `opened`: the underlying spark-icon-button doesn't see changes in the `src` attribute bound to `src` in the enclosing element.