Fix PackageUriResolver.restoreAbsolute on Windows

Fix PackageUriResolver.restoreAbsolute on Windows

Previously, this method attempted to create a package URI by appending
part of a filesystem path to "package:"; on Windows this resulted in
nonsense URIs like "package:foo\bar.dart".

Now we transform the filesystem paths to "file:" URIs before doing other
computations; this normalizes the path separator to "/", so we can
safely use part of the resulting URIs to construct a "package:" URI.

R=brianwilkerson@google.com

Committed: https://github.com/dart-lang/sdk/commit/4e41dc0d66d240e20a5e470b66d9071ffad96094

[Paul Berry, 2015-10-20 16:46:05]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-10-20 16:48:05]

LGTM

[Paul Berry, 2015-10-20 16:56:28]

Committed patchset #1 (id:1) manually as
4e41dc0d66d240e20a5e470b66d9071ffad96094 (presubmit successful).