Add support for ^ constraints in pub.

sdk/lib/_internal/pub/test/validator/sdk_constraint_test.dart

Index: sdk/lib/_internal/pub/test/validator/sdk_constraint_test.dart
diff --git a/sdk/lib/_internal/pub/test/validator/sdk_constraint_test.dart b/sdk/lib/_internal/pub/test/validator/sdk_constraint_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4efb822037812bc33d33ca2634b05f00682a9310
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/validator/sdk_constraint_test.dart
@@ -0,0 +1,48 @@
+// 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.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:http/http.dart' as http;
+import 'package:http/testing.dart';
+import 'package:path/path.dart' as path;
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../../lib/src/entrypoint.dart';
+import '../../lib/src/validator.dart';
+import '../../lib/src/validator/sdk_constraint.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+
+Validator sdkConstraint(Entrypoint entrypoint) =>
+  new SdkConstraintValidator(entrypoint);
+
+main() {
+  initConfig();
+
+  group('should consider a package valid if it', () {
+    integration('has no SDK constraint', () {
+      d.validPackage.create();
+      expectNoValidationError(sdkConstraint);
+    });
+
+    integration('has an SDK constraint without ^', () {
+      d.dir(appPath, [
+        d.libPubspec("test_pkg", "1.0.0", sdk: ">=1.8.0 <2.0.0")
+      ]).create();
+      expectNoValidationError(sdkConstraint);
+    });
+  });
+
+  test("should consider a package invalid if it has an SDK constraint with "
+      "^", () {
+    d.dir(appPath, [
+      d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
+    ]).create();
+    expect(schedulePackageValidation(sdkConstraint),
+        completion(pairOf(anyElement(contains('">=1.8.0 <2.0.0"')), isEmpty)));
+  });
+}