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

sdk/lib/_internal/pub_generated/test/dart2js/does_not_compile_if_disabled_test.dart

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  integration("does not compile if dart2js is disabled", () {
+    d.dir(
+        appPath,
+        [
+            d.appPubspec(),
+            d.dir(
+                "web",
+                [d.file("main.dart", "void main() => print('hello');")])]).create();
+    pubServe(args: ["--no-dart2js"]);
+    requestShould404("main.dart.js");
+    endPubServe();
+  });
+}