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

sdk/lib/_internal/pub_generated/test/serve/missing_dependency_file_test.dart

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+main() {
+  initConfig();
+  integration("responds with a 404 for a missing files in dependencies", () {
+    d.dir("foo", [d.libPubspec("foo", "0.0.1")]).create();
+    d.dir(appPath, [d.appPubspec({
+        "foo": {
+          "path": "../foo"
+        }
+      })]).create();
+    pubGet();
+    pubServe();
+    requestShould404("packages/foo/nope.dart");
+    requestShould404("dir/packages/foo/nope.dart");
+    endPubServe();
+  });
+}