Fix lowercasing output filenames for dart:3320 as well as fix for co19 test

lib/compiler/implementation/filenames.dart

Index: lib/compiler/implementation/filenames.dart
diff --git a/lib/compiler/implementation/filenames.dart b/lib/compiler/implementation/filenames.dart
index ad086b3242eb636f6261cd3edbbf3a3fb37f5db1..d3f4e7b3ab3baff6a11d3562ffb73defee102b4f 100644
--- a/lib/compiler/implementation/filenames.dart
+++ b/lib/compiler/implementation/filenames.dart
@@ -14,23 +14,11 @@
 // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
 
 String nativeToUriPath(String filename) {
-  if (Platform.operatingSystem != 'windows') return filename;
-  filename = filename.toLowerCase();
-  filename = filename.replaceAll('\\', '/');
-  if (filename.length > 2 && filename[1] == ':') {
-    filename = "/$filename";
-  }
-  return filename;
+  return new Path.fromNative(filename).toString();
 }
 
 String uriPathToNative(String path) {
-  if (Platform.operatingSystem != 'windows') return path;
-  path = path.toLowerCase();
-  if (path.length > 3 && path[0] == '/' && path[2] == ':') {
-    return path.substring(1).replaceAll('/', '\\');
-  } else {
-    return path.replaceAll('/', '\\');
-  }
+  return new Path(path).toNativePath();
 }
 
 Uri getCurrentDirectory() {