The main difference between this version and the old is that the following
three $pipelines will produce the same output (SERVER-6468):
{'a.b': 1} // classic inclusion
{a: {b: 1} // nested inclusion
{'a.b': '$a.b'} // dotted FieldPathExpression
{a: {b: '$a.b'} // nested FieldPathExpression
Additionally support for exclusion of fields other than _id has been
removed for 2.2 (SERVER-6361)
Other related bugs:
SERVER-6177 better error for {a:1, 'a.b':1}
SERVER-6181 support for computed _id
SERVER-6184 support {'a.b': 1, a:{c:1}} in either order
Remaining TODOs:
Re-enable the isSimple() method
See if this can be used more directly by $group to support nested fields (SERVER-6198)
Disabled test for SERVER-5369 because it depends on ability to exclude
arbitrary fields.
