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

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

+import 'package:scheduled_test/scheduled_stream.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  integration("does not compile until its output is requested", () {
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "version": "0.0.1"
+      }),
+          d.dir("web", [d.file("syntax-error.dart", "syntax error")])]).create();
+    var server = pubServe();
+    server.stdout.expect("Build completed successfully");
+    requestShould404("syntax-error.dart.js");
+    server.stdout.expect(
+        emitsLines(
+            "[Info from Dart2JS]:\n" "Compiling myapp|web/syntax-error.dart..."));
+    server.stdout.expect(consumeThrough("Build completed with 1 errors."));
+    endPubServe();
+  });
+}