Remove scanBuiltinLibraries.

sdk/lib/_internal/compiler/implementation/use_unused_api.dart

 // Copyright (c) 2012, 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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 179 matching lines @@
 
 usedByTests() {
   // TODO(ahe): We should try to avoid including API used only for tests. In
   // most cases, such API can be moved to a test library.
   dart2jslib.World world = null;
   dart2jslib.Compiler compiler = null;
   compiler.currentlyInUserCode();
   type_graph_inferrer.TypeGraphInferrer typeGraphInferrer = null;
   source_file_provider.SourceFileProvider sourceFileProvider = null;
   world.hasAnyUserDefinedGetter(null);
-  compiler.importHelperLibrary(null);
   typeGraphInferrer.getCallersOf(null);
   dart_types.Types.sorted(null);
   new dart_types.Types(compiler).copy(compiler);
   new universe.TypedSelector.subclass(null, null);
   new universe.TypedSelector.subtype(null, null);
   new universe.TypedSelector.exact(null, null);
   sourceFileProvider.readStringFromUri(null);
 }
 
 useElements(elements.ClassElement e, elements.Name n) {
@@ skipping 14 matching lines @@
     ..visitExpression(null)
     ..visitFunctionDefinition(null)
     ..visitInvokeStatic(null)
     ..visitLetCont(null)
     ..visitNode(null)
     ..visitParameter(null);
   task
     ..hasIr(null)
     ..getIr(null);
 }