A colon is used as separator between the scheme and id. The logic in
ExternalId.Key#parse(String) that parses an external ID string looks for
the first occurrance of a colon to split the string. This means the
scheme must not contain any colon. Since a scheme is optional also the
id must not contain any colon.
We can't enforce this since internally at Google we violate this
assumption, but we also have our own parse logic and don't rely on the
parse logic in ExternalId.Key#parse(String).
Change-Id: I9a20776c6546eb09562f3da8f8e41506a74ba1e1
Signed-off-by: Edwin Kempin <ekempin@google.com>
207e738cc9