Fix HashMap undefined due to missing import
chore(examples): Add a PolymerJS paper component using attribute mappings
feat(element binder): Bind to Web Component properties

Closes #1277
chore(example): Add paper-checkbox to the paper demo
chore(karma): Restrict the numbers of files watched for faster startup
chore(example): In the Paper example, make the checkbox less confusing
perf(dccd): Drop useless checks

in _recordAdd(), previous could never be null as the list is never empty
(it has at least a marker).
in _recordRemove(), when the list is composed of a single record, it
must be the record we're trying to remove (and we assert that).

Closes #1256
Merge branch 'fix-missing-import-for-hashmap'