Fix --pub-serve's path separator on Windows.

lib/src/runner/browser/server.dart

Index: lib/src/runner/browser/server.dart
diff --git a/lib/src/runner/browser/server.dart b/lib/src/runner/browser/server.dart
index 99e22a9952aabcaf7dcf1bf1f8f32d5a14a209c9..730262edf04658ecd8ea2afa47511e85076dc081 100644
--- a/lib/src/runner/browser/server.dart
+++ b/lib/src/runner/browser/server.dart
@@ -219,8 +219,8 @@ void main() {
 
     var suiteUrl;
     if (_config.pubServeUrl != null) {
-      var suitePrefix = p.withoutExtension(
-          p.relative(path, from: p.join(_root, 'test')));
+      var suitePrefix = p.toUri(p.withoutExtension(
+          p.relative(path, from: p.join(_root, 'test')))).path;
 
       var dartUrl;
       // Polymer generates a bootstrap entrypoint that wraps the entrypoint we