Extract BackendUsageBuilder from BackendUsage

pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart

Index: pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
index 9f091a22a95dc93e0c7028439f65fd84c53981c5..5344604623c2e67f391b7ba72b1f5423f30d11f5 100644
--- a/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/custom_elements_analysis.dart
@@ -10,6 +10,7 @@ import '../universe/use.dart' show StaticUse;
 import '../universe/world_impact.dart'
     show WorldImpact, StagedWorldImpactBuilder;
 import 'backend.dart';
+import 'backend_usage.dart' show BackendUsageBuilder;
 
 /**
  * Support for Custom Elements.
@@ -153,6 +154,8 @@ class CustomElementsAnalysisJoin {
 
   CustomElementsAnalysisJoin(this.backend);
 
+  BackendUsageBuilder get backendUsageBuilder => backend.backendUsageBuilder;
+
   WorldImpact flush() {
     if (!demanded) return const WorldImpact();
     var newActiveClasses = new Set<ClassElement>();
@@ -173,7 +176,7 @@ class CustomElementsAnalysisJoin {
               .registerStaticUse(new StaticUse.foreignUse(constructor));
         }
         escapingConstructors
-            .forEach(backend.backendUsage.registerGlobalDependency);
+            .forEach(backendUsageBuilder.registerGlobalDependency);
         // Force the generaton of the type constant that is the key to an entry
         // in the generated table.
         ConstantValue constant = makeTypeConstant(classElement);