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

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

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  integration("doesn't support invalid commandLineOptions type", () {
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "transformers": [{
+            "\$dart2js": {
+              "commandLineOptions": "foo"
+            }
+          }]
+      }), d.dir("web", [d.file("main.dart", "void main() {}")])]).create();
+    var server = pubServe();
+    requestShould404("main.dart.js");
+    server.stderr.expect(
+        emitsLines(
+            'Build error:\n' 'Transform Dart2JS on myapp|web/main.dart threw error: '
+                'Invalid value for \$dart2js.commandLineOptions: '
+                '"foo" (expected list of strings).'));
+    endPubServe();
+  });
+}