Error message for invalid library name is unclear

[leonsenft]

If a library name in a library directive contains an identifier that begins with a number, the resulting error message is unclear:

library foo.bar.123_baz.qux;
                ^^^^^^^^^^^

Expected to find ';'. [expected_token]

An error message that explains that a library identifier can't begin with a number would be more clear.

An AngularDart client encountered this issue due to their project being hosted under a directory whose name began with a number (which led to the AngularDart compiler emitting this library directive).