For historical reasons, the VM is currently too lenient when parsing library parts. Files that get loaded through a 'part' directive must start with a 'part of libraryname;' clause. The VM so far has not reported an error if the clause is missing.
This change enforces the grammar as the Spec mandates it. Library parts that do not start with 'part of' will no longer compile.
R=srdjan@google.com
Review URL: https://codereview.chromium.org//14791005
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@22278 260f80e4-7a28-3924-810f-c04153c831b5