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PackageGraph was relying on its own knowledge of which AssetCascades were dirty, based on which cascades had assets updated. However, it's possible for a cascade to become dirty due to another cascade's asset being updated, and that case was missed. This change addresses that. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//24460004 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@27904 260f80e4-7a28-3924-810f-c04153c831b5