refactor(collection): use Map instead of Object

modules/facade/src/collection.dart

@@ -9,7 +9,7 @@ class MapWrapper {
   static void set(m, k, v){ m[k] = v; }
   static contains(m, k) => m.containsKey(k);
   static forEach(m, fn) {
-    m.forEach(fn);
+    m.forEach((k,v) => fn(v,k));
   }
 }
 

modules/facade/src/collection.es6

@@ -4,13 +4,11 @@ export var Set = window.Set;
 
 export class MapWrapper {
   static create():Map { return new Map(); }
-  static get(m, k) { return m[k]; }
-  static set(m, k, v) { m[k] = v; }
-  static contains(m, k) { return  m[k] != undefined; }
+  static get(m, k) { return m.get(k); }
+  static set(m, k, v) { m.set(k,v); }
+  static contains(m, k) { return  m.has(k); }
   static forEach(m, fn) {
-    for(var k in m) {
-      fn(k, m[k]);
-    }
+    m.forEach(fn);
   }
 }