Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/lib/src/validator/executable.dart

Index: sdk/lib/_internal/pub_generated/lib/src/validator/executable.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/validator/executable.dart b/sdk/lib/_internal/pub_generated/lib/src/validator/executable.dart
new file mode 100644
index 0000000000000000000000000000000000000000..537edc43635ba7e08a54910f10c412146d317735
--- /dev/null
+++ b/sdk/lib/_internal/pub_generated/lib/src/validator/executable.dart
@@ -0,0 +1,42 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library pub.validator.executable;
+
+import 'dart:async';
+
+import 'package:path/path.dart' as p;
+
+import '../entrypoint.dart';
+import '../validator.dart';
+
+/// Validates that a package's pubspec doesn't contain executables that
+/// reference non-existent scripts.
+class ExecutableValidator extends Validator {
+  ExecutableValidator(Entrypoint entrypoint)
+      : super(entrypoint);
+
+  Future validate() {
+    final completer0 = new Completer();
+    scheduleMicrotask(() {
+      try {
+        var binFiles =
+            entrypoint.root.listFiles(beneath: "bin", recursive: false).map(((path) {
+          return entrypoint.root.relative(path);
+        })).toList();
+        entrypoint.root.pubspec.executables.forEach(((executable, script) {
+          var scriptPath = p.join("bin", "$script.dart");
+          if (binFiles.contains(scriptPath)) return;
+          warnings.add(
+              'Your pubspec.yaml lists an executable "$executable" that '
+                  'points to a script "$scriptPath" that does not exist.');
+        }));
+        completer0.complete();
+      } catch (e, s) {
+        completer0.completeError(e, s);
+      }
+    });
+    return completer0.future;
+  }
+}