Remove ES 2015 and ES 2016 polyfill sets

All of the browsers we currently support, as specified by the
`browserslist` key in package.json, fully support ES 2015 and ES 2016.

The only exception I'm aware of is that Edge Legacy v17 and v18 do not
support using string regex search/match methods with custom (non-regex)
types via `Symbol.{match, search, replace}`. That's not something we've
ever used AFAIK.

src/boot/boot.js

@@ -5,8 +5,6 @@ import { requiredPolyfillSets } from './polyfills';
  */
 const commonPolyfills = [
   // ES APIs
-  'es2015',
-  'es2016',
   'es2017',
   'es2018',
 

src/boot/polyfills/es2015.js

-// ES2015
-//
-// nb. The imports which add entire classes (Promise, Set etc.) will also add
-// all features for later ES years, so this can result in some duplication with
-// bundles for later ES years.
-import 'core-js/es/promise';
-import 'core-js/es/map';
-import 'core-js/es/number';
-import 'core-js/es/set';
-import 'core-js/es/symbol';
-import 'core-js/es/array/fill';
-import 'core-js/es/array/find';
-import 'core-js/es/array/find-index';
-import 'core-js/es/array/from';
-import 'core-js/es/object/assign';
-import 'core-js/es/string/includes';
-import 'core-js/es/string/ends-with';
-import 'core-js/es/string/starts-with';

src/boot/polyfills/es2016.js

-import 'core-js/es/array/includes';

src/boot/polyfills/index.js

@@ -22,37 +22,6 @@ function hasMethods(obj, ...methods) {
  * if the browser has the functionality natively available.
  */
 const needsPolyfill = {
-  es2015: () => {
-    // Check for new objects in ES2015.
-    if (
-      typeof Promise !== 'function' ||
-      typeof Map !== 'function' ||
-      typeof Set !== 'function' ||
-      typeof Symbol !== 'function'
-    ) {
-      return true;
-    }
-
-    // Check for new methods on existing objects in ES2015.
-    const objMethods = [
-      [Array, 'from'],
-      [Array.prototype, 'fill', 'find', 'findIndex'],
-      [Object, 'assign'],
-      [String.prototype, 'startsWith', 'endsWith'],
-    ];
-    for (let [obj, ...methods] of objMethods) {
-      if (!hasMethods(obj, ...methods)) {
-        return true;
-      }
-    }
-
-    return false;
-  },
-
-  es2016: () => {
-    return !hasMethods(Array.prototype, 'includes');
-  },
-
   es2017: () => {
     return !hasMethods(Object, 'entries', 'values');
   },

src/boot/polyfills/test/index-test.js

@@ -24,14 +24,6 @@ describe('boot/polyfills/index', () => {
     });
 
     [
-      {
-        set: 'es2015',
-        providesMethod: [Object, 'assign'],
-      },
-      {
-        set: 'es2016',
-        providesMethod: [Array.prototype, 'includes'],
-      },
       {
         set: 'es2017',
         providesMethod: [Object, 'entries'],

src/boot/test/boot-test.js

@@ -30,7 +30,7 @@ describe('bootstrap', function () {
   function runBoot() {
     const assetNames = [
       // Polyfills
-      'scripts/polyfills-es2015.bundle.js',
+      'scripts/polyfills-es2017.bundle.js',
 
       // Annotation layer
       'scripts/annotator.bundle.js',
@@ -110,7 +110,7 @@ describe('bootstrap', function () {
 
     it('loads polyfills if required', () => {
       fakePolyfills.requiredPolyfillSets.callsFake(sets =>
-        sets.filter(s => s.match(/es2015/))
+        sets.filter(s => s.match(/es2017/))
       );
 
       runBoot();
@@ -120,7 +120,7 @@ describe('bootstrap', function () {
       );
       assert.called(fakePolyfills.requiredPolyfillSets);
       assert.deepEqual(polyfillsLoaded, [
-        assetUrl('scripts/polyfills-es2015.bundle.1234.js'),
+        assetUrl('scripts/polyfills-es2017.bundle.1234.js'),
       ]);
     });
   });
@@ -153,7 +153,7 @@ describe('bootstrap', function () {
 
     it('loads polyfills if required', () => {
       fakePolyfills.requiredPolyfillSets.callsFake(sets =>
-        sets.filter(s => s.match(/es2015/))
+        sets.filter(s => s.match(/es2017/))
       );
 
       runBoot();
@@ -163,7 +163,7 @@ describe('bootstrap', function () {
       );
       assert.called(fakePolyfills.requiredPolyfillSets);
       assert.deepEqual(polyfillsLoaded, [
-        assetUrl('scripts/polyfills-es2015.bundle.1234.js'),
+        assetUrl('scripts/polyfills-es2017.bundle.1234.js'),
       ]);
     });
   });