Revert "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
deleted file mode 100644
index 0fe9ac87742986e5660d29822f7a1dd6ed734f00..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/validator/sdk_constraint.dart
+++ /dev/null
@@ -1,45 +0,0 @@
-// 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;
-  }
-}