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

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

Index: sdk/lib/_internal/pub_generated/lib/src/validator/sdk_constraint.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/validator/sdk_constraint.dart b/sdk/lib/_internal/pub_generated/lib/src/validator/sdk_constraint.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0fe9ac87742986e5660d29822f7a1dd6ed734f00
--- /dev/null
+++ b/sdk/lib/_internal/pub_generated/lib/src/validator/sdk_constraint.dart
@@ -0,0 +1,45 @@
+// Copyright (c) 2014, 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.sdk_constraint;
+
+import 'dart:async';
+
+import 'package:pub_semver/pub_semver.dart';
+
+import '../entrypoint.dart';
+import '../log.dart' as log;
+import '../package.dart';
+import '../validator.dart';
+
+/// A validator that validates that a package's SDK constraint doesn't use the
+/// "^" syntax.
+class SdkConstraintValidator extends Validator {
+  SdkConstraintValidator(Entrypoint entrypoint)
+      : super(entrypoint);
+
+  Future validate() {
+    final completer0 = new Completer();
+    scheduleMicrotask(() {
+      try {
+        var constraint = entrypoint.root.pubspec.environment.sdkVersion;
+        join0() {
+          errors.add(
+              "^ version constraints aren't allowed for SDK constraints since "
+                  "older versions of pub don't support them.\n" "Expand it manually instead:\n"
+                  "\n" "environment:\n" "  sdk: \">=${constraint.min} <${constraint.max}\"");
+          completer0.complete();
+        }
+        if (!constraint.toString().startsWith("^")) {
+          completer0.complete(null);
+        } else {
+          join0();
+        }
+      } catch (e, s) {
+        completer0.completeError(e, s);
+      }
+    });
+    return completer0.future;
+  }
+}