Add a fast case to Array.join when all the elements and the separator are fla...

src/arm/full-codegen-arm.cc

Index: src/arm/full-codegen-arm.cc
===================================================================
--- src/arm/full-codegen-arm.cc	(revision 5857)
+++ src/arm/full-codegen-arm.cc	(working copy)
@@ -2772,6 +2772,13 @@
 }
 
 
+void FullCodeGenerator::EmitFastAsciiArrayJoin(ZoneList<Expression*>* args) {
+  __ LoadRoot(r0, Heap::kUndefinedValueRootIndex);
+  context()->Plug(r0);
+  return;
+}
+
+
 void FullCodeGenerator::VisitCallRuntime(CallRuntime* expr) {
   Handle<String> name = expr->name();
   if (name->length() > 0 && name->Get(0) == '_') {