Remove Promise.cast; update Promise.resolve

A while ago Promise.resolve took on Promise.cast's behavior, and Promise.cast then died. This bit me pretty hard today when switching between Node 0.11's promises (for which that change is present) and Traceur's under Node 0.10 (which still have the old resolve).

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1293

Closes #1293.