Replace size getter in SimpleMap class with regularly updated property

Author: lbeschastny

src/simple-map.js

@@ -1,13 +1,10 @@
 export class SimpleMap {
     constructor () {
+        this.size = 0;
         this.keys = [];
         this.values = [];
     }
 
-    get size () {
-        return this.keys.length;
-    }
-
     get (key) {
         const index = this.keys.indexOf(key);
 
@@ -17,6 +14,7 @@ export class SimpleMap {
     set (key, value) {
         this.keys.push(key);
         this.values.push(value);
+        this.size = this.keys.length;
 
         return value;
     }