Check PackageMapUriResolver constructor arguments.

pkg/analyzer/lib/src/util/asserts.dart

+// Copyright (c) 2015, 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 engine.utilities.asserts;
+
+/**
+ * Ensures that the given [value] is not null.
+ * Otherwise throws an [ArgumentError].
+ * An optional [description] is used in the error message.
+ */
+void notNull(Object value, [String description]) {
+  if (value == null) {
+    if (description == null) {
+      throw new ArgumentError('Must not be null');
+    } else {
+      throw new ArgumentError('Must not be null: $description');
+    }
+  }
+}