Load builtin helper libraries on demand.

sdk/lib/_internal/compiler/implementation/js_backend/backend.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.
 
 part of js_backend;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked;
 
   JavaScriptItemCompilationContext()
@@ skipping 1413 matching lines @@
       // TODO(ahe): This is a little hacky, but I'll have to rewrite this when
       // implementing @MirrorsUsed anyways.
       compiler.constantHandler.compiledConstants.addAll(
           compiler.metadataHandler.compiledConstants);
       compiler.metadataHandler.compiledConstants.clear();
     }
     if (mustRetainMetadata) hasRetainedMetadata = true;
     return mustRetainMetadata;
   }
 
-  void onLibraryScanned(LibraryElement library, Uri uri) {
+  void onLibraryLoaded(LibraryElement library, Uri uri) {
     if (uri == Uri.parse('dart:_js_mirrors')) {
       disableTreeShakingMarker =
           library.find(const SourceString('disableTreeShaking'));
       preserveMetadataMarker =
           library.find(const SourceString('preserveMetadata'));
     } else if (uri == Uri.parse('dart:_js_names')) {
       preserveNamesMarker =
           library.find(const SourceString('preserveNames'));
     }
   }
@@ skipping 72 matching lines @@
   }
 }
 
 /// Records that [type] is used by [user.element].
 class Dependency {
   final DartType type;
   final TreeElements user;
 
   const Dependency(this.type, this.user);
 }