BREAKING CHANGE: enforce part of directive

BREAKING CHANGE: enforce part of directive

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

Committed: https://code.google.com/p/dart/source/detail?r=22278

[hausner, 2013-05-01 20:25:34]

Many files, but the change is actually tiny.

[srdjan, 2013-05-01 20:28:13]

lgtm

[hausner, 2013-05-01 20:47:03]

Committed patchset #1 manually as r22278 (presubmit successful).