[test] Fix typo in assertInstanceOf in mjsunit.js.

test/mjsunit/mjsunit.js

Index: test/mjsunit/mjsunit.js
diff --git a/test/mjsunit/mjsunit.js b/test/mjsunit/mjsunit.js
index 78c97528756a51eee5ec794f3b71fd783b66f52b..ffde5ad99671057611eee9f8355737431bc21e47 100644
--- a/test/mjsunit/mjsunit.js
+++ b/test/mjsunit/mjsunit.js
@@ -399,7 +399,7 @@ var assertMatches;
       if (typeof actualConstructor === "function") {
         actualTypeName = actualConstructor.name || String(actualConstructor);
       }
-      failWithmessage("Object <" + PrettyPrint(obj) + "> is not an instance of <" +
+      failWithMessage("Object <" + PrettyPrint(obj) + "> is not an instance of <" +
                (type.name || type) + ">" +
                (actualTypeName ? " but of <" + actualTypeName + ">" : ""));
     }