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

sdk/lib/_internal/pub_generated/test/downgrade/dry_run_does_not_apply_changes_test.dart

+import 'package:path/path.dart' as path;
+import 'package:scheduled_test/scheduled_test.dart';
+import '../../lib/src/io.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("--dry-run shows report but does not apply changes", () {
+    servePackages((builder) {
+      builder.serve("foo", "1.0.0");
+      builder.serve("foo", "2.0.0");
+    });
+    d.appDir({
+      "foo": "2.0.0"
+    }).create();
+    pubGet();
+    d.appDir({
+      "foo": "any"
+    }).create();
+    schedule(() {
+      deleteEntry(path.join(sandboxDir, appPath, "packages"));
+    });
+    pubDowngrade(
+        args: ["--dry-run"],
+        output: allOf(
+            [contains("< foo 1.0.0"), contains("Would change 1 dependency.")]));
+    d.dir(
+        appPath,
+        [
+            d.matcherFile("pubspec.lock", contains("2.0.0")),
+            d.nothing("packages")]).validate();
+  });
+}