WIP: Add custom platform library.

sdk/lib/_internal/js_runtime/lib/custom_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/custom_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/custom_patch.dart b/sdk/lib/_internal/js_runtime/lib/custom_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f1e09bac7bfe5be0d82d8eaa60aa0f3541d701b7
--- /dev/null
+++ b/sdk/lib/_internal/js_runtime/lib/custom_patch.dart
@@ -0,0 +1,9 @@
+import 'dart:_js_helper' show patch;
+import 'dart:_foreign_helper' show JS;
+
+@patch
+void externalMethod(String argument) {
+  // This is a call to the 'magic' JS function that generates JavaScript using
+  // the second argument as a template.
+  JS('void', 'print("External method in dart:custom: " + #)', argument);
+}