Properly convert paths to package root URIs.

Closes #44

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1055513003

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 * Add support for `shelf` `0.6.0`.
 
+* Fix a "failed to load" bug on Windows.
+
 ### 0.12.0-beta.2
 
 * Rename the package to `test`. The `unittest` package will continue to exist

lib/src/util/io.dart

@@ -100,7 +100,7 @@ String packageRootFor(String path, [String override]) {
     throw new LoadException(path, "Directory $packageRoot does not exist.");
   }
 
-  return packageRoot;
+  return p.toUri(packageRoot).toString();
 }
 
 /// The library name must be globally unique, or the wrong library path may be

pubspec.yaml

 name: test
-version: 0.12.0-beta.2
+version: 0.12.0-dev
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test