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

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

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+main() {
+  initConfig();
+  integration("does not serve .dart files in release mode", () {
+    d.dir(
+        "foo",
+        [d.libPubspec("foo", "0.0.1"), d.dir("lib", [d.file("foo.dart", """
+            library foo;
+            foo() => 'foo';
+            """)])]).create();
+    d.dir(appPath, [d.appPubspec({
+        "foo": {
+          "path": "../foo"
+        }
+      }),
+          d.dir("lib", [d.file("lib.dart", "lib() => print('hello');")]),
+          d.dir("web", [d.file("file.dart", """
+            import 'package:foo/foo.dart';
+            main() => print('hello');
+            """),
+            d.dir("sub", [d.file("sub.dart", "main() => 'foo';")])])]).create();
+    pubServe(shouldGetFirst: true, args: ["--mode", "release"]);
+    requestShould404("file.dart");
+    requestShould404("packages/myapp/lib.dart");
+    requestShould404("packages/foo/foo.dart");
+    requestShould404("sub/sub.dart");
+    endPubServe();
+  });
+}