Fix mutation of childNodes array

js/id/core/graph.js

@@ -84,6 +84,8 @@ iD.Graph.prototype = {
             nodes[i] = this.entity(entity.nodes[i]);
         }
 
+        if (iD.debug) Object.freeze(nodes);
+
         this._childNodes[entity.id] = nodes;
         return this._childNodes[entity.id];
     },

js/id/geo/multipolygon.js

@@ -87,7 +87,7 @@ iD.geo.joinWays = function(array, graph) {
     while (array.length) {
         member = array.shift();
         current = [member];
-        current.nodes = nodes = resolve(member);
+        current.nodes = nodes = resolve(member).slice();
         joined.push(current);
 
         while (array.length && _.first(nodes) !== _.last(nodes)) {