Add Array.concat builtin for the most common case.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 61534d3571dcb8111a66c4ae45d618d130f58d39..72f7cf4b266b95cfc835118669885f2c63d86e2c 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -1297,6 +1297,7 @@ static Object* Runtime_SpecialArrayFunctions(Arguments args) {
   InstallBuiltin(holder, "unshift", Builtins::ArrayUnshift);
   InstallBuiltin(holder, "slice", Builtins::ArraySlice);
   InstallBuiltin(holder, "splice", Builtins::ArraySplice);
+  InstallBuiltin(holder, "concat", Builtins::ArrayConcat);
 
   return *holder;
 }