Fix a bug in type assertions involving String supertypes.

dart/lib/compiler/implementation/ssa/codegen.dart

Index: dart/lib/compiler/implementation/ssa/codegen.dart
diff --git a/dart/lib/compiler/implementation/ssa/codegen.dart b/dart/lib/compiler/implementation/ssa/codegen.dart
index 446650771825bda19d4374ec396891120b1b03fd..258a85b6aca743042f8034e0757249a61b46567b 100644
--- a/dart/lib/compiler/implementation/ssa/codegen.dart
+++ b/dart/lib/compiler/implementation/ssa/codegen.dart
@@ -2444,6 +2444,7 @@ class SsaCodeGenerator implements HVisitor, HBlockInformationVisitor {
       } else if (element == compiler.intClass) {
         helper = const SourceString('intTypeCheck');
       } else if (Elements.isStringSupertype(element, compiler)) {
+        additionalArgument = compiler.namer.operatorIs(element);
         if (nativeCheck) {
           helper = const SourceString('stringSuperNativeTypeCheck');
         } else {