Cache area

js/id/core/entity.js

@@ -95,6 +95,12 @@ iD.Entity.prototype = {
         return this.extent(resolver).intersects(extent);
     },
 
+    area: function(resolver, path) {
+        return resolver.transient(this, 'area', function() {
+            return path.area(this);
+        });
+    },
+
     hasInterestingTags: function() {
         return _.keys(this.tags).some(function(key) {
             return key != 'attribution' &&

js/id/svg/areas.js

@@ -37,12 +37,12 @@ iD.svg.Areas = function(projection) {
             if (multipolygon = iD.geo.isSimpleMultipolygonOuterMember(entity, graph)) {
                 areas[multipolygon.id] = {
                     entity: multipolygon.mergeTags(entity.tags),
-                    area: Math.abs(path.area(entity))
+                    area: Math.abs(entity.area(graph, path))
                 };
             } else if (!areas[entity.id]) {
                 areas[entity.id] = {
                     entity: entity,
-                    area: Math.abs(path.area(entity))
+                    area: Math.abs(entity.area(graph, path))
                 };
             }
         }