Fixed assert causing crash when attempting to load a library from an invalid path.

tests/language/vm/regress_27201_lib.dart

+// Copyright (c) 2016, 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 import_lib;
+
+final foo = 1;
+var someVar = 3;
+var _privateVar;
+
+int get someGetter => 2;
+
+void set someSetter(int val) {}
+
+int someFunc() => 0;
+
+class SomeClass {
+}
+
+typedef int Func(Object a);