expose dynGet() {to be used in all.equal.*()}

git-svn-id: https://svn.r-project.org/R/trunk@66634 00db46b3-68df-0310-9c12-caf00c1e9a41
belongs to rev 66629

git-svn-id: https://svn.r-project.org/R/trunk@66635 00db46b3-68df-0310-9c12-caf00c1e9a41