fix dart2js warnings when compiling polymer_expressions

fix dart2js warnings when compiling polymer_expressions
it might be a dart2js bug, but I'm not sure the generic adds anything here, so removed it.

R=justinfagnani@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27267

[Jennifer Messerly, 2013-09-06 19:07:01]

the errors looked like:

out/ReleaseIA32/packages/polymer_expressions/visitor.dart:29:11: Warning:
"ParenthesizedExpression" is not assignable to "E".
    visit(e);

[justinfagnani, 2013-09-06 19:11:54]

lgtm

[Jennifer Messerly, 2013-09-06 20:33:31]

Committed patchset #1 manually as r27267 (presubmit successful).