Extract BackendUsageBuilder from BackendUsage

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

 // Copyright (c) 2013, 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.
 
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 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.
  *
  * The support for custom elements the compiler builds a table that maps the
  * custom element class's [Type] to the interceptor for the class and the
  * constructor(s) for the class.
  *
  * We want the table to contain only the custom element classes used, and we
  * want to avoid resolving and compiling constructors that are not used since
@@ skipping 123 matching lines @@
   bool allClassesSelected = false;
 
   // Did we see a demand for the data?
   bool demanded = false;
 
   // ClassesOutput: classes requiring metadata.
   final activeClasses = new Set<ClassElement>();
 
   CustomElementsAnalysisJoin(this.backend);
 
+  BackendUsageBuilder get backendUsageBuilder => backend.backendUsageBuilder;
+
   WorldImpact flush() {
     if (!demanded) return const WorldImpact();
     var newActiveClasses = new Set<ClassElement>();
     for (ClassElement classElement in instantiatedClasses) {
       bool isNative = backend.isNative(classElement);
       bool isExtension =
           !isNative && backend.nativeData.isNativeOrExtendsNative(classElement);
       // Generate table entries for native classes that are explicitly named and
       // extensions that fix our criteria.
       if ((isNative && selectedClasses.contains(classElement)) ||
           (isExtension &&
               (allClassesSelected || selectedClasses.contains(classElement)))) {
         newActiveClasses.add(classElement);
         Iterable<ConstructorElement> escapingConstructors =
             computeEscapingConstructors(classElement);
         for (ConstructorElement constructor in escapingConstructors) {
           impactBuilder
               .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);
         backend.computeImpactForCompileTimeConstant(
             constant, impactBuilder, false);
         backend.addCompileTimeConstantForEmission(constant);
       }
     }
     activeClasses.addAll(newActiveClasses);
     instantiatedClasses.removeAll(newActiveClasses);
@@ skipping 24 matching lines @@
           result.add(member);
         }
       }
     }
 
     classElement.forEachMember(selectGenerativeConstructors,
         includeBackendMembers: false, includeSuperAndInjectedMembers: false);
     return result;
   }
 }