Remove BackendClasses and JavaScriptBackendClasses.

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 f3fdc407c644b5c2e71587e0ccf267ed6d74fbc4..cb37946307672b288f3e362bc32fb5a6f785de1c 100644
--- a/pkg/compiler/lib/src/inferrer/node_tracer.dart
+++ b/pkg/compiler/lib/src/inferrer/node_tracer.dart
@@ -397,7 +397,7 @@ abstract class TracerVisitor implements TypeInformationVisitor {
    */
   bool isParameterOfListAddingMethod(Element element) {
     if (!element.isRegularParameter) return false;
-    if (element.enclosingClass != compiler.backend.backendClasses.listClass) {
+    if (element.enclosingClass != compiler.commonElements.jsArrayClass) {
       return false;
     }
     String name = element.enclosingElement.name;
@@ -411,7 +411,7 @@ abstract class TracerVisitor implements TypeInformationVisitor {
    */
   bool isParameterOfMapAddingMethod(Element element) {
     if (!element.isRegularParameter) return false;
-    if (element.enclosingClass != compiler.backend.backendClasses.mapClass) {
+    if (element.enclosingClass != compiler.commonElements.mapLiteralClass) {
       return false;
     }
     String name = element.enclosingElement.name;