dart2js: Don't add properties to native classes that are not needed.

dart2js: Don't add properties to native classes that are not needed.

This has no impact on the output, and almost no impact on the compilation time.
However, it makes it clearer, that unneeded native classes really are not needed, and don't need their nativeSpecProperty either.

I want to refactor this code in another CL, but I think it's cleaner if this if-check is already in place now, and not merged into a bigger change.

R=sra@google.com

Committed: https://code.google.com/p/dart/source/detail?r=43096

[floitsch, 2015-01-22 17:14:48]

floitsch@google.com changed reviewers:
+ sra@google.com

[sra1, 2015-01-22 17:30:31]

lgtm

[floitsch, 2015-01-23 10:35:53]

Committed patchset #1 (id:1) manually as 43096 (presubmit successful).