- Make sure that externals are patched for dart2js.

sdk/lib/_internal/js_runtime/lib/io_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/io_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/io_patch.dart b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
index 23f4708da2ff1947ca20b0d23ed5c40705ac5efb..350ab9693224ccc5ae0bd87470dca023b1085c95 100644
--- a/sdk/lib/_internal/js_runtime/lib/io_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/io_patch.dart
@@ -233,6 +233,10 @@ class _Platform {
     throw new UnsupportedError("Platform._packageRoot");
   }
   @patch
+  static String _packageConfig() {
+    throw new UnsupportedError("Platform._packageConfig");
+  }
+  @patch
   static _environment() {
     throw new UnsupportedError("Platform._environment");
   }