Allow tests to specify a package root.
Allow tests to specify a package root.

Use a shared package root if none is specified.

Committed: https://code.google.com/p/dart/source/detail?r=15064

Review URL: https://codereview.chromium.org//11359187

git-svn-id: 965eb99b257a0350f91a5ab8607b3dea57271703@15070 260f80e4-7a28-3924-810f-c04153c831b5

seaneagan/args,ronlobo/args,iansf/args,dart-lang/args