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

tests/language/part_of_uri_test.dart

Index: tests/language/part_of_uri_test.dart
diff --git a/tests/language/part_of_uri_test.dart b/tests/language/part_of_uri_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4c313a0cccd0d54bd21e62c251e7cfb426321feb
--- /dev/null
+++ b/tests/language/part_of_uri_test.dart
@@ -0,0 +1,14 @@
+// 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.
+
+// No library declaration
+part "part_of_uri_part.dart";  // declares bar1, baz1
+part "part_of_uri_part2.dart";  // declares bar2, baz2
+
+const foo = 'foo';
+const qux = "$baz1$baz2";
+
+main() {
+  if (!identical(qux, "foopart21foopart12")) throw "Fail: $qux";
+}