Revert "Remove deprecated mappedBy."
Revert "Remove deprecated mappedBy."

This reverts commit 18575.

Review URL: https://codereview.chromium.org//12207205

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@18576 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/pub,cjkao/dart-pub,dart-lang/pub,maxwellpeterson-wf/pub,computmaxer/pub