fix(@angular-devkit/build-angular): downlevel and optimize locale data

Locale data is now transformed to be compatible with the ECMAScript level of the application bundles.  The locale data is also optimized to remove comments and unnecessary whitespace.

Fixes: #17497

Author: clydin

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

@@ -634,7 +634,7 @@ export async function inlineLocales(options: InlineOptions) {
       // If locale data is provided, load it and prepend to file
       const localeDataPath = i18n.locales[locale]?.dataPath;
       if (localeDataPath) {
-        localeDataContent = await loadLocaleData(localeDataPath, true);
+        localeDataContent = await loadLocaleData(localeDataPath, true, options.es5);
       }
     }
 
@@ -753,7 +753,7 @@ async function inlineLocalesDirect(ast: ParseResult, options: InlineOptions) {
       let localeDataSource: Source | null = null;
       const localeDataPath = i18n.locales[locale] && i18n.locales[locale].dataPath;
       if (localeDataPath) {
-        const localeDataContent = await loadLocaleData(localeDataPath, true);
+        const localeDataContent = await loadLocaleData(localeDataPath, true, options.es5);
         localeDataSource = new OriginalSource(localeDataContent, path.basename(localeDataPath));
       }
 
@@ -870,19 +870,36 @@ function findLocalizePositions(
   return positions;
 }
 
-async function loadLocaleData(path: string, optimize: boolean): Promise<string> {
+async function loadLocaleData(path: string, optimize: boolean, es5: boolean): Promise<string> {
   // The path is validated during option processing before the build starts
   const content = fs.readFileSync(path, 'utf8');
 
-  // NOTE: This can be removed once the locale data files are preprocessed in the framework
-  if (optimize) {
-    const result = await terserMangle(content, {
-      compress: true,
-      ecma: 5,
-    });
+  // Downlevel and optimize the data
+  const transformResult = await transformAsync(content, {
+    filename: path,
+    // The types do not include the false option even though it is valid
+    // tslint:disable-next-line: no-any
+    inputSourceMap: false as any,
+    babelrc: false,
+    configFile: false,
+    presets: [
+      [
+        require.resolve('@babel/preset-env'),
+        {
+          bugfixes: true,
+          // IE 9 is the oldest supported browser
+          targets: es5 ? { ie: '9' } : { esmodules: true },
+        },
+      ],
+    ],
+    minified: allowMinify && optimize,
+    compact: !shouldBeautify && optimize,
+    comments: !optimize,
+  });
 
-    return result.code;
+  if (!transformResult || !transformResult.code) {
+    throw new Error(`Unknown error occurred processing bundle for "${path}".`);
   }
 
-  return content;
+  return transformResult.code;
 }

tests/legacy-cli/e2e/tests/i18n/ivy-localize-dl-xliff2.ts

@@ -50,6 +50,18 @@ export async function executeTest() {
     await expectFileToMatch(`${outputPath}/vendor-es5.js`, '.ng.common.locales');
     await expectFileToMatch(`${outputPath}/vendor-es2015.js`, '.ng.common.locales');
 
+    // Verify the locale data is browser compatible
+    await expectToFail(() => expectFileToMatch(`${outputPath}/vendor-es5.js`, /\bconst\b/));
+    await expectFileToMatch(`${outputPath}/vendor-es2015.js`, /\bconst\b/);
+
+    // Verify locale data comments are removed in production
+    await expectToFail(() =>
+      expectFileToMatch(`${outputPath}/vendor-es5.js`, '// See angular/tools/gulp-tasks/cldr/extract.js'),
+    );
+    await expectToFail(() =>
+      expectFileToMatch(`${outputPath}/vendor-es2015.js`, '// See angular/tools/gulp-tasks/cldr/extract.js'),
+    );
+
     // Execute Application E2E tests with dev server
     await ng('e2e', `--configuration=${lang}`, '--port=0');