Refactor BackendHelpers to make 'find' methods private.

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

Index: pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
diff --git a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
index 57487179f081d74d350f0662d6a136d2955eeebd..303bd9fc6e6215cc5c972b7a646baa3ae1f00e57 100644
--- a/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
+++ b/pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart
@@ -20,8 +20,10 @@ class CheckedModeHelper {
 
   StaticUse getStaticUse(Compiler compiler) {
     JavaScriptBackend backend = compiler.backend;
+    // TODO(johnniwinther): Refactor this to avoid looking up directly in the
+    // js helper library but instead access helpers directly on backend helpers.
     return new StaticUse.staticInvoke(
-        backend.helpers.findHelper(name), callStructure);
+        backend.helpers.jsHelperLibrary.find(name), callStructure);
   }
 
   CallStructure get callStructure => CallStructure.ONE_ARG;