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

pkg/front_end/lib/src/fasta/kernel/kernel_procedure_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_procedure_builder;
 
 import 'package:kernel/ast.dart' show
     Arguments,
     AsyncMarker,
     Constructor,
@@ skipping 28 matching lines @@
 
 import '../errors.dart' show
     internalError;
 
 import '../messages.dart' show
     warning;
 
 import '../loader.dart' show
     Loader;
 
-import '../util/relativize.dart' show
-    relativizeUri;
-
 import 'kernel_builder.dart' show
     Builder,
     ClassBuilder,
     ConstructorReferenceBuilder,
     FormalParameterBuilder,
     KernelFormalParameterBuilder,
     KernelLibraryBuilder,
     KernelTypeBuilder,
     KernelTypeVariableBuilder,
     MetadataBuilder,
@@ skipping 120 matching lines @@
   final ConstructorReferenceBuilder redirectionTarget;
 
   KernelProcedureBuilder(
       List<MetadataBuilder> metadata,
       int modifiers, KernelTypeBuilder returnType, String name,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals, this.actualAsyncModifier,
       ProcedureKind kind, KernelLibraryBuilder compilationUnit, int charOffset,
       [String nativeMethodName, this.redirectionTarget])
       : procedure = new Procedure(null, kind, null,
-            fileUri: relativizeUri(compilationUnit?.fileUri)),
+            fileUri: compilationUnit?.relativeFileUri),
         super(metadata, modifiers, returnType, name, typeVariables, formals,
             compilationUnit, charOffset, nativeMethodName);
 
   ProcedureKind get kind => procedure.kind;
 
   AsyncMarker get asyncModifier => actualAsyncModifier;
 
   Statement get body {
     if (actualBody == null && redirectionTarget == null && !isAbstract &&
         !isExternal) {
@@ skipping 128 matching lines @@
         }
       }
       initializers.add(initializer..parent = constructor);
       initializers.add(superInitializer);
       return;
     }
     initializers.add(initializer);
     initializer.parent = constructor;
   }
 }