Creates a local version of the Dart assembler code, before being
merged into our code base. The goal of these files is to track code as
it is moved from the Dart implementation into our code base.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1394613002 .