Drop stage-2 and avoid using static class properties.

Author: ghengeveld

.babelrc

@@ -1,4 +1,4 @@
 {
-  "presets": [["env", { "loose": true }], "stage-2", "react"],
+  "presets": [["env", { "loose": true }], "react"],
   "ignore": ["node_modules", "stories.js"]
 }

package.json

@@ -50,7 +50,6 @@
     "babel-preset-env": "1.7.0",
     "babel-preset-flow": "6.23.0",
     "babel-preset-react": "6.24.1",
-    "babel-preset-stage-2": "6.24.1",
     "eslint": "5.11.1",
     "eslint-config-prettier": "3.3.0",
     "eslint-plugin-jest": "22.1.2",

src/Async.spec.js

@@ -107,8 +107,11 @@ describe("Async", () => {
 
   test("re-runs the promise when the value of 'watch' changes", () => {
     class Counter extends React.Component {
-      state = { count: 0 }
-      inc = () => this.setState(state => ({ count: state.count + 1 }))
+      constructor(props) {
+        super(props)
+        this.state = { count: 0 }
+        this.inc = () => this.setState(state => ({ count: state.count + 1 }))
+      }
       render() {
         return (
           <div>
@@ -133,8 +136,11 @@ describe("Async", () => {
 
   test("re-runs the promise when 'watchFn' returns truthy", () => {
     class Counter extends React.Component {
-      state = { count: 0 }
-      inc = () => this.setState(state => ({ count: state.count + 1 }))
+      constructor(props) {
+        super(props)
+        this.state = { count: 0 }
+        this.inc = () => this.setState(state => ({ count: state.count + 1 }))
+      }
       render() {
         return (
           <div>

src/useAsync.spec.js

@@ -103,8 +103,11 @@ describe("useAsync", () => {
 
   test("re-runs the promise when the value of 'watch' changes", () => {
     class Counter extends React.Component {
-      state = { count: 0 }
-      inc = () => this.setState(state => ({ count: state.count + 1 }))
+      constructor(props) {
+        super(props)
+        this.state = { count: 0 }
+        this.inc = () => this.setState(state => ({ count: state.count + 1 }))
+      }
       render() {
         return (
           <div>
@@ -128,8 +131,11 @@ describe("useAsync", () => {
 
   test("re-runs the promise when 'watchFn' returns truthy", () => {
     class Counter extends React.Component {
-      state = { count: 0 }
-      inc = () => this.setState(state => ({ count: state.count + 1 }))
+      constructor(props) {
+        super(props)
+        this.state = { count: 0 }
+        this.inc = () => this.setState(state => ({ count: state.count + 1 }))
+      }
       render() {
         return (
           <div>