Address long-standing review comments.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 26191)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -772,11 +772,11 @@
     ensure(jsUnknownJavaScriptObjectClass);
   }
 
-  void registerWrapException(TreeElements elements) {
+  void registerThrowExpression(TreeElements elements) {
+    // We don't know ahead of time whether we will need the throw in a
+    // statement context or an expression context, so we register both
+    // here, even though we may not need the throwExpression helper.
     enqueueInResolution(getWrapExceptionHelper(), elements);
-  }
-
-  void registerThrowExpression(TreeElements elements) {
     enqueueInResolution(getThrowExpressionHelper(), elements);
   }