Implement UriResolver.restoreAbsolute() for DartUriResolver.

pkg/analyzer/lib/src/generated/source.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 analyzer.src.generated.source;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 156 matching lines @@
   DartSdk get dartSdk => _sdk;
 
   @override
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (!isDartUri(uri)) {
       return null;
     }
     return _sdk.mapDartUri(uri.toString());
   }
 
+  @override
+  Uri restoreAbsolute(Source source) {
+    Source dartSource = _sdk.fromFileUri(source.uri);
+    return dartSource?.uri;
+  }
+
   /**
    * Return `true` if the given URI is a `dart-ext:` URI.
    *
    * @param uriContent the textual representation of the URI being tested
    * @return `true` if the given URI is a `dart-ext:` URI
    */
   static bool isDartExtUri(String uriContent) =>
       uriContent != null && uriContent.startsWith(_DART_EXT_SCHEME);
 
   /**
@@ skipping 569 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]);
 
   /**
    * Return an absolute URI that represents the given [source], or `null` if a
    * valid URI cannot be computed.
    *
    * The computation should be based solely on [source.fullName].
    */
   Uri restoreAbsolute(Source source) => null;
 }