Fix assertion in Dartium loading
Fix assertion in Dartium loading

A library may be marked as “loaded with error” multiple times it it
consists of parts. Fix an assertion that caused some tests in Dartium
to fail.

Fixes issue 19758.

R=regis@google.com

Review URL: https://codereview.chromium.org//369573006

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@37958 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk