Generate method to get assigned oneof field about protobuf.dart

[sigurdm]

Fixed by #166 .

[osa1]

As @sigurdm notes oneof support was implemented in #166. The get...Case methods in Java are which... methods in Dart. Example:

syntax = "proto2";

message Simple {
  oneof colors {
    int32 red = 2;
    int32 green = 4;
  }
}

Simple s1 = Simple.create();
assert(s1.whichColors() == Simple_Colors.notSet);

s1.red = 1;
assert(s1.whichColors() == Simple_Colors.red);

s1.green = 1;
assert(s1.whichColors() == Simple_Colors.green);