Add m.prop (#2268)

Fixes #2095

util/prop.js

@@ -0,0 +1,9 @@
+"use strict"
+
+module.exports = function (store) {
+	return {
+		get: function() { return store },
+		toJSON: function() { return store },
+		set: function(value) { return store = value }
+	}
+}

util/tests/index.html

@@ -8,8 +8,10 @@
 		<script src="../../ospec/ospec.js"></script>
 
 		<script src="../../util/withAttr.js"></script>
+		<script src="../../util/prop.js"></script>
 		<script src="test-withAttr.js"></script>
-		
+		<script src="test-prop.js"></script>
+
 		<script>require("../../ospec/ospec").run()</script>
 	</body>
 </html>

util/tests/test-prop.js

@@ -0,0 +1,16 @@
+"use strict"
+
+var o = require("../../ospec/ospec")
+var prop = require("../../util/prop")
+
+o.spec("prop", function() {
+	o("works", function() {
+		var p = prop(1)
+
+		o(p.get()).equals(1)
+		o(p.toJSON()).equals(1)
+		o(p.set(2)).equals(2)
+		o(p.get()).equals(2)
+		o(p.toJSON()).equals(2)
+	})
+})