should fix issue from http://stackoverflow.com/questions/24918894

- I think the cause of your error message were missing imports (see diff below).
- You should use the Polymer 0.12.0-dev release (to get dev-releases a specific dependency constraint is necessary)
- in my DartEditor config I have 'remove leading spaces on save' activated. Several changed lines are caused by removed spaces only.
- one end tag had underlines instead of dashes
- The 'Tools Output' window showed warnings about these missing imports
It is a bit unfortunate that they are scroll out on the top very quick (see also https://code.google.com/p/dart/issues/detail?id=20118)
- You use a `<core-card>` element. I wasn't able to find its definition (seems not to exist) but that doesn't cause troubles so far