Regression test for #37.

Also clean up example.

BUG=https://github.com/dart-lang/dart_style/issues/37
R=pquitslund@google.com

Review URL: https://chromiumcodereview.appspot.com//779373002
Preserve a blank line before a comment inside an expression.

Along with previous changes, fixes #23.

BUG=https://github.com/dart-lang/dart_style/issues/23
R=pquitslund@google.com

Review URL: https://chromiumcodereview.appspot.com//781153004
Handle comments preceding operators correctly. Fix #72.

Also, don't emit trailing spaces in some corner cases.

BUG=https://github.com/dart-lang/dart_style/issues/72
R=pquitslund@google.com

Review URL: https://chromiumcodereview.appspot.com//794443002
remove unused imports