Add support for ^ constraints in pub.

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
new file mode 100644
index 0000000000000000000000000000000000000000..e409f52f5e6dedd18afa5faadcc800cdcdb54490
--- /dev/null
+++ b/sdk/lib/_internal/pub/lib/src/validator/sdk_constraint.dart
@@ -0,0 +1,34 @@
+// 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() 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}\"");
+  }
+}