goog.msg static fields need extra init statement in gents

[rkirov]

The following input:

class C {

}
C.message = goog.msg("...");

gents is smart enough to not move this to a 'static' field, because that breaks goog.msg. However, that's not a valid TS. One answer is to have it emit:

class C {
  static message = '';
}
C.message = goog.msg("...");

[ukrukarg]

Actual working solution:
Class C {
/** @desc */
static message = goog.msg("...");
}