The contract for AbsTraversalForExpr calls for us to interpret an
expression as if it were traversal syntax. Traversal syntax does not have
the special keywords "null", "true" and "false", so we must interpret
these as TraverseRoot rather than as literal values.
Previously this wasn't working because the parser converted these to
literals too early. To make this work properly, we implement
AbsTraversalForExpr on literal expressions and effectively "undo" the
parser's re-interpretation of these keywords to back out to the original
keyword strings.
We also rework how object keys are handled so that we wait until eval time
to decide whether to interpret the key expression as an unquoted literal
string. This allows us to properly support AbsTraversalForExpr on keys
in object constructors, bypassing the string-interpretation behavior in
that case.
cc8b14cf45