Fix most implementations of UriResolver.restoreAbsolute.

pkg/analyzer/test/file_system/resource_uri_resolver_test.dart

 // Copyright (c) 2014, 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.resource_uri_resolver;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 36 matching lines @@
     var uri = new Uri(scheme: 'https', path: '127.0.0.1/test.dart');
     Source source = resolver.resolveAbsolute(uri);
     expect(source, isNull);
   }
 
   void test_restoreAbsolute() {
     var uri = new Uri(scheme: 'file', path: '/test.dart');
     Source source = resolver.resolveAbsolute(uri);
     expect(source, isNotNull);
     expect(resolver.restoreAbsolute(source), uri);
+    expect(
+        resolver.restoreAbsolute(
+            new NonExistingSource(source.fullName, null, null)),
+        uri);
   }
 }