Make test.dart run web tests on Windows.

frog/library.dart

Index: frog/library.dart
===================================================================
--- frog/library.dart	(revision 3505)
+++ frog/library.dart	(working copy)
@@ -63,6 +63,7 @@
     if (filename.startsWith('/')) return filename;
     if (filename.startsWith('file:///')) return filename;
     if (filename.startsWith('http://')) return filename;
+    if (const RegExp('^[a-zA-Z]:/').hasMatch(filename)) return filename;
     return joinPaths(sourceDir, filename);
   }