Progress toward modular compilation - step 0, introduce compilation summaries.

Review URL: https://chromiumcodereview.appspot.com//12543022
Switch dwc to use pathos.

Review URL: https://chromiumcodereview.appspot.com//12730015
Update pubspec: seems that we already published the package in pub so I'm just
making our version match.

This reverts commit 34ce8fb2e194bb5ae4dfd126088a28e01a50753c.
fixes #398, error when file isn't found

Review URL: https://chromiumcodereview.appspot.com//12723005
Forward dwc options from build.dart

Review URL: https://chromiumcodereview.appspot.com//12472026
Fix typo in comments
merged
Using old CSS errors mechanism and cleaned up parsing/fixup/emit.