Commit some missing code

Author: jfairbank

code/babel/babelrc-latest.js

@@ -0,0 +1,4 @@
+{
+  "presets": ["latest"],
+  "plugins": ["transform-runtime"]
+}

code/events/async-counter.js

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+let counter = 0;
+
+async function listenIncrement() {
+  await click(incrementEl);
+  counter += 1;
+  counterEl.innerHTML = counter;
+  listenIncrement();
+}
+
+listenIncrement();

code/events/callback-counter.js

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+let counter = 0;
+
+function updateCounter(n) {
+  counter += n;
+  counterEl.innerHTML = counter;
+}
+
+incrementEl.addEventListener('click', () => {
+  updateCounter(1);
+});
+
+decrementEl.addEventListener('click', () => {
+  updateCounter(-1);
+});

code/events/observable-counter.js

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+const source = Observable.merge(
+  Observable.fromEvent(incrementBtn, 'click').mapTo(1),
+  Observable.fromEvent(decrementBtn, 'click').mapTo(-1),
+);
+
+source
+  .scan((acc, curr) => acc + curr, 0)
+  .subscribe((counter) => {
+    counterEl.innerHTML = counter;
+  });

code/installation.sh

@@ -0,0 +1,7 @@
+$ npm install --save-dev \
+    babel-core \
+    babel-cli \
+    babel-preset-latest \
+    babel-plugin-transform-runtime
+
+$ npm install --save babel-runtime