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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
index 38c17eee428d3e774541459d72cc3bd9f7b9d7ba..7e023ab1312a3f6900a9f44a7deca3129b408500 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
@@ -46,9 +46,6 @@ import '../messages.dart' show
 import '../loader.dart' show
     Loader;
 
-import '../util/relativize.dart' show
-    relativizeUri;
-
 import 'kernel_builder.dart' show
     Builder,
     ClassBuilder,
@@ -189,7 +186,7 @@ class KernelProcedureBuilder extends KernelFunctionBuilder {
       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);