Run client-side JS through Babel if requested

Gruntfile.js

@@ -25,12 +25,28 @@ module.exports = function (grunt) {
     grunt.initConfig({
         pkg: grunt.file.readJSON('package.json'),
         compile: {
-            'android': {},
-            'ios': {},
-            'osx': {},
-            'windows': { useWindowsLineEndings: true },
-            'browser': {},
-            'electron': {}
+            android: {
+                targets: 'Android >= 4.4, ChromeAndroid >= 30'
+            },
+            ios: {
+                targets: 'iOS >= 9'
+            },
+            osx: {
+                // TODO: add actual targets based on platform support
+                // {} means transform any ES2015+ code to ES5
+                targets: {}
+            },
+            windows: {
+                targets: 'Explorer >= 11',
+                useWindowsLineEndings: true
+            },
+            browser: {
+                // {} means transform any ES2015+ code to ES5
+                targets: {}
+            },
+            electron: {
+                targets: 'Electron >= 4'
+            }
         },
         clean: ['pkg']
     });
@@ -45,6 +61,7 @@ module.exports = function (grunt) {
 
         build({
             platformName: this.target,
+            babel: { targets: this.data.targets },
             platformVersion: grunt.option('platformVersion') ||
                              require(platformPkgPath).version,
             extraModules: collectModules(platformModulesPath)

build-tools/build.js

@@ -17,6 +17,7 @@
  * under the License.
  */
 
+const babel = require('@babel/core');
 const execa = require('execa');
 const bundle = require('./bundle');
 const scripts = require('./scripts');
@@ -34,9 +35,28 @@ module.exports = function build (userConfig) {
         .then(([ scripts, modules, commitId ]) => {
             Object.assign(config, { commitId });
             return bundle(scripts, modules, config);
-        });
+        })
+        .then(babelTransform(config));
 };
 
 function getCommitId () {
     return execa.stdout('git', ['rev-parse', 'HEAD'], { cwd: pkgRoot });
 }
+
+function babelTransform (config) {
+    const targets = (config.babel || {}).targets;
+    if (!targets) return x => x;
+
+    const babelOpts = {
+        retainLines: true,
+        presets: [
+            ['@babel/preset-env', {
+                targets,
+                // Do not force strict mode as that breaks some platforms
+                modules: false
+            }]
+        ]
+    };
+    return inputCode => babel.transformAsync(inputCode, babelOpts)
+        .then(result => result.code);
+}

package.json

@@ -69,6 +69,8 @@
     }
   ],
   "dependencies": {
+    "@babel/core": "^7.6.4",
+    "@babel/preset-env": "^7.6.3",
     "execa": "^1.0.0",
     "fs-extra": "^8.0.1",
     "globby": "^9.2.0"

test/build.js

@@ -25,6 +25,8 @@ function buildCordovaJsTestBundle (bundlePath) {
         platformName: 'test',
         platformVersion: 'N/A',
         extraModules: collectTestBuildModules(),
+        // {} means transform any ES2015+ code to ES5
+        babel: { targets: {} },
         preprocess (f) {
             // Do not instrument our test dummies
             if (f.path.includes('src/legacy-exec/test/')) return f;