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

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

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("doesn't change git dependencies", () {
+    ensureGit();
+    d.git('foo.git', [d.libDir('foo'), d.libPubspec('foo', '1.0.0')]).create();
+    d.appDir({
+      "foo": {
+        "git": "../foo.git"
+      }
+    }).create();
+    pubGet();
+    d.dir(
+        packagesPath,
+        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
+    d.git(
+        'foo.git',
+        [d.libDir('foo', 'foo 2'), d.libPubspec('foo', '1.0.0')]).commit();
+    pubDowngrade();
+    d.dir(
+        packagesPath,
+        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
+  });
+}