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

pkg/analyzer/lib/src/summary/public_namespace_computer.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 analyzer.src.summary.public_namespace_visitor;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/summary/format.dart';
 import 'package:analyzer/src/summary/idl.dart';
 
@@ skipping 160 matching lines @@
   }
 
   @override
   visitFunctionTypeAlias(FunctionTypeAlias node) {
     addNameIfPublic(node.name.name, ReferenceKind.typedef,
         node.typeParameters?.typeParameters?.length ?? 0);
   }
 
   @override
   visitPartDirective(PartDirective node) {
-    parts.add(node.uri.stringValue);
+    parts.add(node.uri.stringValue ?? '');
   }
 
   @override
   visitVariableDeclaration(VariableDeclaration node) {
     String name = node.name.name;
     addNameIfPublic(name, ReferenceKind.topLevelPropertyAccessor, 0);
     if (!node.isFinal && !node.isConst) {
       addNameIfPublic('$name=', ReferenceKind.topLevelPropertyAccessor, 0);
     }
   }
 }