Reduce use of relativize and unsort methods.

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

 // Copyright (c) 2016, 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 fasta.source_class_builder;
 
 import 'package:front_end/src/fasta/builder/class_builder.dart'
     show ClassBuilder;
 
 import 'package:front_end/src/fasta/source/source_library_builder.dart'
@@ skipping 15 matching lines @@
         KernelTypeBuilder,
         KernelTypeVariableBuilder,
         LibraryBuilder,
         MetadataBuilder,
         Scope,
         TypeVariableBuilder,
         compareProcedures;
 
 import '../dill/dill_member_builder.dart' show DillMemberBuilder;
 
-import '../util/relativize.dart' show relativizeUri;
-
 Class initializeClass(
-    Class cls, String name, LibraryBuilder parent, int charOffset) {
+    Class cls, String name, KernelLibraryBuilder parent, int charOffset) {
   cls ??= new Class(name: name);
-  cls.fileUri ??= relativizeUri(parent.fileUri);
+  cls.fileUri ??= parent.library.fileUri;
   if (cls.fileOffset == TreeNode.noOffset) {
     cls.fileOffset = charOffset;
   }
   return cls;
 }
 
 class SourceClassBuilder extends KernelClassBuilder {
   final Class cls;
 
   final List<ConstructorReferenceBuilder> constructorReferences;
@@ skipping 103 matching lines @@
   }
 
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder library, ClassBuilder currentClass) {
     scope.forEach((name, builder) {
       builder.prepareInitializerInference(library, this);
     });
   }
 }