Fixed <spark-menu>

The <shadow> was somehow causing the element to crash: substituted <shadow> with <content>, which is nearly identical in the context, and it worked.
Fixed <spark-selector>

valueToIndex() was returning a 0 while the list of items was empty, causing a crash. Tried to fix that, and also added caching for items to avoid polling every time. The fix is still a little like poking in the dark, but at least it doesn't crash.
Merge pull request #471 from dart-lang/fix-crashing-polymer-widgets

Fixed crashing polymer widgets