Reduce use of relativize and unsort methods.

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

Index: pkg/front_end/lib/src/fasta/source/source_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
index 7b6c27af767857bab2c7da7f69504d748a316942..11acdf99338ac1657ad84d1ae2c4b1623b251777 100644
--- a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
@@ -33,12 +33,10 @@ import '../kernel/kernel_builder.dart'
 
 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;
   }