Replace ClosedWorld.backend with ClosedWorld.backendClasses

pkg/compiler/lib/src/inferrer/node_tracer.dart

Index: pkg/compiler/lib/src/inferrer/node_tracer.dart
diff --git a/pkg/compiler/lib/src/inferrer/node_tracer.dart b/pkg/compiler/lib/src/inferrer/node_tracer.dart
index cc890bf5f7bcff18793ba94dfdcd714caf3f0780..058d36ddc5cc33664930350ede385f1f4f459035 100644
--- a/pkg/compiler/lib/src/inferrer/node_tracer.dart
+++ b/pkg/compiler/lib/src/inferrer/node_tracer.dart
@@ -333,7 +333,8 @@ abstract class TracerVisitor<T extends TypeInformation>
    */
   bool isParameterOfListAddingMethod(Element element) {
     if (!element.isRegularParameter) return false;
-    if (element.enclosingClass != compiler.backend.listImplementation) {
+    if (element.enclosingClass !=
+        compiler.backend.backendClasses.listImplementation) {
       return false;
     }
     Element method = element.enclosingElement;
@@ -349,7 +350,8 @@ abstract class TracerVisitor<T extends TypeInformation>
    */
   bool isParameterOfMapAddingMethod(Element element) {
     if (!element.isRegularParameter) return false;
-    if (element.enclosingClass != compiler.backend.mapImplementation) {
+    if (element.enclosingClass !=
+        compiler.backend.backendClasses.mapImplementation) {
       return false;
     }
     Element method = element.enclosingElement;