Remove support for missing part-of tag.

tests/compiler/dart2js_extra/source_mapping_crash_source.dart

Index: tests/compiler/dart2js_extra/source_mapping_crash_source.dart
diff --git a/tests/compiler/dart2js_extra/source_mapping_crash_source.dart b/tests/compiler/dart2js_extra/source_mapping_crash_source.dart
index 825360f25dbbe87a3bd056cf271dfcd1f78bb7d5..78f15e38d6335359a2466fa78b86dee8f43f912e 100644
--- a/tests/compiler/dart2js_extra/source_mapping_crash_source.dart
+++ b/tests/compiler/dart2js_extra/source_mapping_crash_source.dart
@@ -2,6 +2,8 @@
 // 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 source_mapping_crash_test;
+
 /*******************************************************************************
  * Long comment to make positions in this file exceed those in
  * 'source_mapping_crash_test.dart'.