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

Review URL: https://codereview.chromium.org/1416863003 .

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