Add implicit constraints from pub onto stack_trace and source_maps.

These packages are used in pub's plugin isolate, so pub needs to have
a dependency on them to ensure their APIs don't change out from under
it.

R=rnystrom@google.com
BUG=19574

Review URL: https://codereview.chromium.org//356523003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@37751 260f80e4-7a28-3924-810f-c04153c831b5