fix: implement SimpleMap without using getters/setters

Squashed commit of the following:

commit ea31b3a8b0a60f2e428a61fdf28f6cc0f8d469e2
Author: Gajus Kuizinas <gajus@gajus.com>
Date:   Sat Sep 10 10:59:27 2016 +0100

    refactor: remove SimpleMap factory

commit 9549afdab039fb151387c65214e78edaef3a3ada
Merge: 1cb161d fb31781
Author: Gajus Kuizinas <gajus@gajus.com>
Date:   Sat Sep 10 10:54:33 2016 +0100

    Merge branch 'simple-map-ie8-fix' of https://github.com/lbeschastny/react-css-modules into lbeschastny-simple-map-ie8-fix

commit fb31781
Author: Leonid Beschastny <lbeschastny@iknow.travel>
Date:   Mon Aug 29 12:00:17 2016 +0300

    Add createSimpleMap factory function to defer SimpleMap creation

commit 7e6a9f9
Author: Leonid Beschastny <lbeschastny@iknow.travel>
Date:   Mon Aug 29 11:52:35 2016 +0300

    Replace size getter in SimpleMap class with regularly updated property

Author: gajus

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;
     }

tests/simple-map.js

@@ -1,7 +1,9 @@
 import {
     expect
 } from 'chai';
-import {SimpleMap} from './../src/simple-map';
+import {
+		SimpleMap
+} from './../src/simple-map';
 
 const getTests = (map) => {
     return () => {