Replace ClosedWorld.backend with ClosedWorld.backendClasses

pkg/compiler/lib/src/universe/function_set.dart

Index: pkg/compiler/lib/src/universe/function_set.dart
diff --git a/pkg/compiler/lib/src/universe/function_set.dart b/pkg/compiler/lib/src/universe/function_set.dart
index 2aed1f28c6f4cac574f8333e17622128dd21cf7f..c05c7f923795ce9b12f65342f9b727c38a0a6b0c 100644
--- a/pkg/compiler/lib/src/universe/function_set.dart
+++ b/pkg/compiler/lib/src/universe/function_set.dart
@@ -287,7 +287,7 @@ class FullFunctionSetQuery implements FunctionSetQuery {
           ClassElement cls = element.enclosingClass;
           return [cls]..addAll(closedWorld.mixinUsesOf(cls));
         }).map((cls) {
-          if (closedWorld.backend.isNullImplementation(cls)) {
+          if (closedWorld.backendClasses.nullImplementation == cls) {
             return const TypeMask.empty();
           } else if (closedWorld.isInstantiated(cls.declaration)) {
             return new TypeMask.nonNullSubclass(cls.declaration, closedWorld);