Optimize out checkInt, checkNum and checkString

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

Index: pkg/compiler/lib/src/js_backend/backend_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_helpers.dart b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
index dc5bb125993360269a9a850b666a834355a452f1..f85e9d08152583fe2364b1fbe8677dbb3e7cda94 100644
--- a/pkg/compiler/lib/src/js_backend/backend_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_helpers.dart
@@ -457,6 +457,15 @@ class BackendHelpers {
     return findHelper('throwConcurrentModificationError');
   }
 
+  Element get checkInt => _checkInt ??= findHelper('checkInt');
+  Element _checkInt;
+
+  Element get checkNum => _checkNum ??= findHelper('checkNum');
+  Element _checkNum;
+
+  Element get checkString => _checkString ??= findHelper('checkString');
+  Element _checkString;
+
   Element get stringInterpolationHelper {
     return findHelper('S');
   }