Read package:browser .js files from barback rather than from packages directory.
Read package:browser .js files from barback rather than from packages directory.

The previous code assumes that the packages/ directory exists. This will no
longer be the case if the package-spec DEP is implemented. Also resolves a
breaking edge-case: if a pubspec.lock exists but the packages directory has
been deleted, pub build will terminate with a file not found.

BUG=
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1000193004

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@44468 260f80e4-7a28-3924-810f-c04153c831b5

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