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

sdk/lib/_internal/pub_generated/test/run/nonexistent_dependency_test.dart

+import '../../lib/src/exit_codes.dart' as exit_codes;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration('Errors if the script is in an unknown package.', () {
+    d.dir(appPath, [d.appPubspec()]).create();
+    var pub = pubRun(args: ["foo:script"]);
+    pub.stderr.expect(
+        'Could not find package "foo". Did you forget to add a ' 'dependency?');
+    pub.shouldExit(exit_codes.DATA);
+  });
+}