fix(@angular-devkit/build-angular): generate ES5 code in ES5 bundles for default browserslist configuration

Author: alan-agius4

packages/angular_devkit/build_angular/src/utils/process-bundle.ts

@@ -119,6 +119,17 @@ export async function process(options: ProcessBundleOptions): Promise<ProcessBun
   let downlevelCode;
   let downlevelMap;
   if (downlevel) {
+    const {supportedBrowsers: targets = []} = options;
+
+    // todo: revisit this in version 10, when we update our defaults browserslist
+    // Without this workaround bundles will not be downlevelled because Babel doesn't know handle to 'op_mini all'
+    // See: https://github.com/babel/babel/issues/11155
+    if (Array.isArray(targets) && targets.includes('op_mini all')) {
+      targets.push('ie_mob 11');
+    } else if ('op_mini' in targets) {
+      targets['ie_mob'] = '11';
+    }
+
     // Downlevel the bundle
     const transformResult = await transformAsync(sourceCode, {
       filename,
@@ -132,7 +143,7 @@ export async function process(options: ProcessBundleOptions): Promise<ProcessBun
         require.resolve('@babel/preset-env'),
         {
           // browserslist-compatible query or object of minimum environment versions to support
-          targets: options.supportedBrowsers,
+          targets,
           // modules aren't needed since the bundles use webpack's custom module loading
           modules: false,
           // 'transform-typeof-symbol' generates slower code