Store the async-await compiled pub code directly in the repo.

sdk/lib/_internal/pub_generated/test/cache/add/already_cached_test.dart

+library pub_tests;
+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+main() {
+  initConfig();
+  integration('does nothing if the package is already cached', () {
+    servePackages((builder) {
+      builder.serve("foo", "1.2.3");
+    });
+    schedulePub(
+        args: ["cache", "add", "foo"],
+        output: 'Downloading foo 1.2.3...');
+    schedulePub(
+        args: ["cache", "add", "foo"],
+        output: 'Already cached foo 1.2.3.');
+    d.cacheDir({
+      "foo": "1.2.3"
+    }).validate();
+  });
+}