Patched the spark_widgets imports to work around a Polymer shortcoming

See https://github.com/dart-lang/spark/issues/500.

Changes in spark_widgets themselves are not marked with a "// BUG" because they are good ones overall, and they didn't go as far as the others (the ".../packages/spark_widgets..." part appeared to be unnecessary).
Merge pull request #502 from ussuri/fix-spark_widgets-imports

Patched the spark_widgets imports to work around a Polymer shortcoming