Use precomputed uriBytes while computing cycle signature.

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/src/base/api_signature.dart';
 import 'package:front_end/src/base/performace_logger.dart';
@@ skipping 299 matching lines @@
     var signatureBuilder = new ApiSignature();
     signatureBuilder.addBytes(_salt);
     Set<FileState> transitiveFiles = cycle.libraries
         .map((library) => library.transitiveFiles)
         .expand((files) => files)
         .toSet();
     signatureBuilder.addInt(transitiveFiles.length);
 
     // Append API signatures of transitive files.
     for (var file in transitiveFiles) {
-      signatureBuilder.addString(file.uri.toString());
+      signatureBuilder.addBytes(file.uriBytes);
       // TODO(scheglov): Stop using content hashes here, when Kernel stops
       // copying methods of mixed-in classes.
       // https://github.com/dart-lang/sdk/issues/29881
       if (hasMixinApplication) {
         signatureBuilder.addBytes(file.contentHash);
       } else {
         signatureBuilder.addBytes(file.apiSignature);
       }
     }
 
@@ skipping 53 matching lines @@
 
   _LibraryCycleResult(this.cycle, this.signature, this.kernelLibraries);
 }
 
 @visibleForTesting
 class _TestView {
   /// The list of [LibraryCycle]s compiled for the last delta.
   /// It does not include libraries which were read from the cache.
   final List<LibraryCycle> compiledCycles = [];
 }