Move registerBackend* to BackendUsage

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

Index: pkg/compiler/lib/src/js_backend/type_variable_handler.dart
diff --git a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
index 046eb428caf5eb8d6f173365af5a5470a5f9a200..b4c62e7fc5ca036e3adcaa75e2300226f897c31e 100644
--- a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
+++ b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
@@ -80,11 +80,11 @@ class TypeVariableHandler {
               "Class '$_typeVariableClass' should only have one constructor");
         }
         _typeVariableConstructor = _typeVariableClass.constructors.head;
-        _backend.impactTransformer.registerBackendStaticUse(
+        _backend.backendUsage.registerBackendStaticUse(
             impactBuilderForResolution, _typeVariableConstructor);
-        _backend.impactTransformer.registerBackendInstantiation(
+        _backend.backendUsage.registerBackendInstantiation(
             impactBuilderForResolution, _typeVariableClass);
-        _backend.impactTransformer.registerBackendStaticUse(
+        _backend.backendUsage.registerBackendStaticUse(
             impactBuilderForResolution, _backend.helpers.createRuntimeType);
         _seenClassesWithTypeVariables = true;
       }