Shoehorn dart2js into the SDK.

dart/lib/compiler/implementation/filenames.dart

Index: dart/lib/compiler/implementation/filenames.dart
diff --git a/dart/lib/compiler/implementation/filenames.dart b/dart/lib/compiler/implementation/filenames.dart
index 82ff21c5b05309cc4b70873a494cdac466d6bdcc..2c76740484af51681b300474ea2a7c773cb6cfe5 100644
--- a/dart/lib/compiler/implementation/filenames.dart
+++ b/dart/lib/compiler/implementation/filenames.dart
@@ -27,9 +27,9 @@ 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);
+    return path.substring(1).replaceAll('/', '\\');
   } else {
-    return path;
+    return path.replaceAll('/', '\\');
   }
 }