dart2js kernel: generate `other==null` header for operator== methods

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

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index a0a744ebb9ad76083707b41859a1aeb1f983be9b..e587bbfc06d18e308f6785f4cef188ae7eb93ff1 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -706,7 +706,7 @@ class JavaScriptBackend {
     return class_;
   }
 
-  bool operatorEqHandlesNullArgument(FunctionElement operatorEqfunction) {
+  bool operatorEqHandlesNullArgument(FunctionEntity operatorEqfunction) {
     return specialOperatorEqClasses.contains(operatorEqfunction.enclosingClass);
   }