FIX: Composer preview on IE11 (#7970)

Add the Array.from polyfill for IE11. This is required to support the transpiled ES6 spread syntex generated by babel: https://babeljs.io/docs/en/caveats/

app/assets/javascripts/polyfills.js

@@ -225,4 +225,95 @@ if (!String.prototype.padEnd) {
   };
 }
 
+// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from
+// Production steps of ECMA-262, Edition 6, 22.1.2.1
+if (!Array.from) {
+  Array.from = (function() {
+    var toStr = Object.prototype.toString;
+    var isCallable = function(fn) {
+      return typeof fn === "function" || toStr.call(fn) === "[object Function]";
+    };
+    var toInteger = function(value) {
+      var number = Number(value);
+      if (isNaN(number)) {
+        return 0;
+      }
+      if (number === 0 || !isFinite(number)) {
+        return number;
+      }
+      return (number > 0 ? 1 : -1) * Math.floor(Math.abs(number));
+    };
+    var maxSafeInteger = Math.pow(2, 53) - 1;
+    var toLength = function(value) {
+      var len = toInteger(value);
+      return Math.min(Math.max(len, 0), maxSafeInteger);
+    };
+
+    // The length property of the from method is 1.
+    return function from(arrayLike /*, mapFn, thisArg */) {
+      // 1. Let C be the this value.
+      var C = this;
+
+      // 2. Let items be ToObject(arrayLike).
+      var items = Object(arrayLike);
+
+      // 3. ReturnIfAbrupt(items).
+      if (arrayLike == null) {
+        throw new TypeError(
+          "Array.from requires an array-like object - not null or undefined"
+        );
+      }
+
+      // 4. If mapfn is undefined, then let mapping be false.
+      var mapFn = arguments.length > 1 ? arguments[1] : void undefined;
+      var T;
+      if (typeof mapFn !== "undefined") {
+        // 5. else
+        // 5. a If IsCallable(mapfn) is false, throw a TypeError exception.
+        if (!isCallable(mapFn)) {
+          throw new TypeError(
+            "Array.from: when provided, the second argument must be a function"
+          );
+        }
+
+        // 5. b. If thisArg was supplied, let T be thisArg; else let T be undefined.
+        if (arguments.length > 2) {
+          T = arguments[2];
+        }
+      }
+
+      // 10. Let lenValue be Get(items, "length").
+      // 11. Let len be ToLength(lenValue).
+      var len = toLength(items.length);
+
+      // 13. If IsConstructor(C) is true, then
+      // 13. a. Let A be the result of calling the [[Construct]] internal method
+      // of C with an argument list containing the single item len.
+      // 14. a. Else, Let A be ArrayCreate(len).
+      var A = isCallable(C) ? Object(new C(len)) : new Array(len);
+
+      // 16. Let k be 0.
+      var k = 0;
+      // 17. Repeat, while k < len… (also steps a - h)
+      var kValue;
+      while (k < len) {
+        kValue = items[k];
+        if (mapFn) {
+          A[k] =
+            typeof T === "undefined"
+              ? mapFn(kValue, k)
+              : mapFn.call(T, kValue, k);
+        } else {
+          A[k] = kValue;
+        }
+        k += 1;
+      }
+      // 18. Let putStatus be Put(A, "length", len, true).
+      A.length = len;
+      // 20. Return A.
+      return A;
+    };
+  })();
+}
+
 /* eslint-enable */