Polyfill WeakMap for IE11 (#37)

* Replace WeakMap with object

This update replaces the WeakMap with a regular Object for the cache within
`create-emotion` core. This is to improve IE11 support without adding a (bloated)
polyfill.

* Revert "Replace WeakMap with object"

This reverts commit 563004f.

* Implement a light weight WeakMap polyfill from Google

jest.config.js

@@ -6,6 +6,7 @@ const coverageList = [
   '!src/create-emotion/**/*.{js,jsx}',
   '!src/create-emotion-styled/index.{js,jsx}',
   '!src/**/testHelpers.{js,jsx}',
+  '!src/polyfills/**/*.{js,jsx}',
 ]
 
 module.exports = Object.assign({}, jestConfig, {

src/create-emotion/index.js

@@ -11,6 +11,7 @@ import {
 } from './utils'
 import StyleSheet from './sheet'
 import type {PrefixOption, ClassNameArg} from './utils'
+import '../polyfills/WeakMap'
 
 type StylisPlugins = Function[] | null | Function
 

src/polyfills/WeakMap.js

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012 The Polymer Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style
+ * license that can be found in the LICENSE file.
+ */
+
+if (typeof WeakMap === 'undefined') {
+  ;(function() {
+    var defineProperty = Object.defineProperty
+    var counter = Date.now() % 1e9
+
+    var WeakMap = function() {
+      this.name = '__st' + ((Math.random() * 1e9) >>> 0) + (counter++ + '__')
+    }
+
+    WeakMap.prototype = {
+      set: function(key, value) {
+        var entry = key[this.name]
+        if (entry && entry[0] === key) entry[1] = value
+        else
+          defineProperty(key, this.name, {
+            value: [key, value],
+            writable: true,
+          })
+        return this
+      },
+      get: function(key) {
+        var entry
+        return (entry = key[this.name]) && entry[0] === key
+          ? entry[1]
+          : undefined
+      },
+      delete: function(key) {
+        var entry = key[this.name]
+        if (!entry) return false
+        var hasValue = entry[0] === key
+        entry[0] = entry[1] = undefined
+        return hasValue
+      },
+      has: function(key) {
+        var entry = key[this.name]
+        if (!entry) return false
+        return entry[0] === key
+      },
+    }
+
+    window.WeakMap = WeakMap
+  })()
+}