Fix most implementations of UriResolver.restoreAbsolute.

pkg/analyzer/lib/src/generated/source_io.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 engine.source.io;
 
 import 'dart:collection';
 
 import 'engine.dart';
 import 'java_core.dart';
@@ skipping 265 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     if (!isFileUri(uri)) {
       return null;
     }
     return new FileBasedSource(
         new JavaFile.fromUri(uri), actualUri != null ? actualUri : uri);
   }
 
   @override
   Uri restoreAbsolute(Source source) {
-    if (source is FileBasedSource) {
-      return new Uri.file(source.fullName);
-    }
-    return null;
+    return new Uri.file(source.fullName);
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }
@@ skipping 253 matching lines @@
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }