Right now, only field_attach_view() and field_attach_prepare_view() functions make sure they are working with a BC entity, the others have to be called with one, so the caller needs to make sure.
It would be easier to convert and later on remove it again when the other field attach functions would ensure this, as there are dozens if not hundreds of direct calls t those functions in the field tests, that I am converting in.
PASSED: [[SimpleTest]]: [MySQL] 55,471 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 54,604 pass(es).
FAILED: [[SimpleTest]]: [MySQL] 54,244 pass(es), 150 fail(s), and 10 exception(s).