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

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

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import '../serve/utils.dart';
+main() {
+  initConfig();
+  integration("generates unminified JS when not in release mode", () {
+    d.dir(
+        appPath,
+        [
+            d.appPubspec(),
+            d.dir(
+                "web",
+                [d.file("main.dart", "void main() => print('hello');")])]).create();
+    pubServe(args: ["--mode", "whatever"]);
+    requestShouldSucceed("main.dart.js", isUnminifiedDart2JSOutput);
+    endPubServe();
+  });
+}