fix unused import hints in src/generated/source.dart

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';
-import 'package:analyzer/file_system/physical_file_system.dart';
-import 'package:analyzer/source/package_map_resolver.dart';
 import 'package:analyzer/src/context/source.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/java_core.dart';
 import 'package:analyzer/src/generated/java_engine.dart';
 import 'package:analyzer/src/generated/java_io.dart' show JavaFile;
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk;
 import 'package:analyzer/src/generated/source_io.dart' show FileBasedSource;
-import 'package:analyzer/src/generated/utilities_dart.dart' as utils;
 import 'package:analyzer/task/model.dart';
 import 'package:package_config/packages.dart';
 import 'package:path/path.dart' as pathos;
 
 /**
  * A function that is used to visit [ContentCache] entries.
  */
 typedef void ContentCacheVisitor(String fullPath, int stamp, String contents);
 
 /**
@@ skipping 905 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;
 }