Fix for crash when part directive uri is has null value.

pkg/analyzer/test/src/summary/resynthesize_common.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 test.src.serialization.elements_test;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 4327 matching lines @@
     addSource('/b.dart', 'part of my.lib;');
     checkLibrary('library my.lib; part "a.dart"; part "b.dart";');
   }
 
   test_parts_invalidUri() {
     allowMissingFiles = true;
     addSource('/foo/bar.dart', 'part of my.lib;');
     checkLibrary('library my.lib; part "foo/";');
   }
 
+  test_parts_invalidUri_nullStringValue() {
+    allowMissingFiles = true;
+    addSource('/foo/bar.dart', 'part of my.lib;');
+    checkLibrary(r'''
+library my.lib;
+part "${foo}/bar.dart";
+''');
+  }
+
   test_propagated_type_refers_to_closure() {
     checkLibrary('''
 void f() {
   var x = () => 0;
   var y = x;
 }
 ''');
   }
 
   test_setter_documented() {
@@ skipping 531 matching lines @@
       fail('Unexpectedly tried to get unlinked summary for $uri');
     }
     return serializedUnit;
   }
 
   @override
   bool hasLibrarySummary(String uri) {
     return true;
   }
 }