fasta: Cache relativizeUri(fileUri) on LibraryBuilder and ClassBuilder (or rather it's base class)

pkg/front_end/lib/src/fasta/kernel/kernel_field_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.kernel_field_builder;
 
 import 'package:kernel/ast.dart' show
     Expression,
     Field,
     Library,
     Name;
 
 import 'kernel_builder.dart' show
     Builder,
     FieldBuilder,
     KernelTypeBuilder,
     MetadataBuilder;
 
-import '../util/relativize.dart' show
-    relativizeUri;
-
 class KernelFieldBuilder extends FieldBuilder<Expression> {
   final Field field;
   final List<MetadataBuilder> metadata;
   final KernelTypeBuilder type;
 
   KernelFieldBuilder(this.metadata, this.type, String name, int modifiers,
       Builder compilationUnit, int charOffset)
       : field =
-            new Field(null, fileUri: relativizeUri(compilationUnit?.fileUri))
+            new Field(null, fileUri: compilationUnit?.relativeFileUri)
                 ..fileOffset = charOffset,
         super(name, modifiers, compilationUnit, charOffset);
 
   void set initializer(Expression value) {
     field.initializer = value
         ..parent = field;
   }
 
   Field build(Library library) {
     field.name ??= new Name(name, library);
     if (type != null) {
       field.type = type.build();
     }
     bool isInstanceMember = !isStatic && !isTopLevel;
     return field
         ..isFinal = isFinal
         ..isConst = isConst
         ..hasImplicitGetter = isInstanceMember
         ..hasImplicitSetter = isInstanceMember && !isConst && !isFinal
         ..isStatic = !isInstanceMember;
   }
 
   Field get target => field;
 }