fix dart2js warnings in polymer_expressions pkg

fix dart2js warnings in polymer_expressions pkg

BUG=
R=sigmund@google.com

Committed: https://code.google.com/p/dart/source/detail?r=28627

[Siggi Cherem (dart-lang), 2013-10-11 23:22:59]

lgtm, thank you!

[Siggi Cherem (dart-lang), 2013-10-11 23:23:22]

btw - could we add a analyzer-clean test for polymer_expressions?

[Jennifer Messerly, 2013-10-14 19:46:21]

PTAL, additional cleanups.

I tried to turn on @static-clean as you requested, but am blocked by this bug
https://code.google.com/p/dart/issues/detail?id=14094

[Jennifer Messerly, 2013-10-14 19:48:43]

https://codereview.chromium.org/26318004/diff/6001/pkg/polymer_expressions/li...
File pkg/polymer_expressions/lib/filter.dart (right):

https://codereview.chromium.org/26318004/diff/6001/pkg/polymer_expressions/li...
pkg/polymer_expressions/lib/filter.dart:16: class _InverseTransformer<V, T>
implements Transformer<V, T> {
note: this isn't intended to change anything, just make it more obvious what's
going on by keeping the meaning of T and V constant with respect to the original
Transformer.

(I thought it might help analyzer, but it doesn't)

[Siggi Cherem (dart-lang), 2013-10-14 21:19:36]

lgtm

[Jennifer Messerly, 2013-10-15 02:54:33]

Committed patchset #2 manually as r28627 (presubmit successful).