Fix instanceof for primitives & string.

Without this change a native type with name="Object"
would return true in an instance of tests against a js boolean, number
or String.

Change-Id: I91e7f32f0d2fd76e842d6c1618160a63e0837aa0

dev/core/super/com/google/gwt/dev/jjs/intrinsic/com/google/gwt/lang/Cast.java

@@ -241,7 +241,7 @@ private static native boolean jsinstanceOf(Object obj, String jsTypeStr) /*-{
 
     if (jsTypeStr == "Object") {
       // TODO(rluble): Handle this case in the compiler side.
-      return true;
+      return (typeof obj) == "object";
     } else if (jsTypeStr == "Array" ) {
       // TODO(rluble): remove array special handling once
       // instanceOf can be customized for native classes.