Start pulling pub from its own repo.

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

Index: sdk/lib/_internal/pub/lib/src/validator/sdk_constraint.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/sdk_constraint.dart b/sdk/lib/_internal/pub/lib/src/validator/sdk_constraint.dart
deleted file mode 100644
index b192b0599f93daf418628e53cceb0aade3e79e71..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub/lib/src/validator/sdk_constraint.dart
+++ /dev/null
@@ -1,30 +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 '../entrypoint.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() async {
-    var constraint = entrypoint.root.pubspec.environment.sdkVersion;
-    if (!constraint.toString().startsWith("^")) return;
-
-    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}\"");
-  }
-}