Make Analyzer, VM and dart2js accept URI strings as part-of library identifier.

tests/language/part_of_uri_part.dart

Index: tests/language/part_of_uri_part.dart
diff --git a/tests/language/part_of_uri_part.dart b/tests/language/part_of_uri_part.dart
new file mode 100644
index 0000000000000000000000000000000000000000..55edadf67a45de6e249203b5034923672feb6207
--- /dev/null
+++ b/tests/language/part_of_uri_part.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2017, 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.
+
+part of "part_of_uri_test.dart";
+
+// Refer to declaration in library and other part.
+const bar1 = "part1";
+const baz1 = "$foo${bar2}1";