chore: add @__PURE__ annotation to ES5 output for uglify

tools/gulp/util/annotate-pure.ts

@@ -0,0 +1,22 @@
+
+const iifeExpression =
+    new RegExp('^(var (\\S+) = )(\\(function \\(\\) \\{\\n(?:    ' +
+        '(?:\\/\\*\\*| \\*|\\*\\/|\\/\\/)[^\\n]*\\n)*    function \\2\\([^\\)]*\\) \\{\\n)', 'mg');
+
+/**
+ * Adds `@__PURE__` annotation comments to IIFEs containing ES5-downleveled classes generated by
+ * TypeScript so that Uglify can tree-shake classes that are not referenced.
+ *
+ * @param fileContent The content of the file for which `@__PURE__` will be added.
+ * @returns The content of the file with `@__PURE__` annotations added.
+ */
+export function addPureAnnotations(fileContent: string) {
+  return fileContent
+      // Prefix downleveled classes w/ the @__PURE__ annotation.
+      .replace(iifeExpression, '$1/*@__PURE__*/$3')
+      // Prefix downleveled classes that extend another class w/ the @__PURE__ annotation
+      .replace(
+          /^(var (\S+) = )(\(function \(_super\) \{\n    __extends\(\2, _super\);\n)/mg,
+          '$1/*@__PURE__*/$3'
+      );
+}

tools/gulp/util/package-build.ts

@@ -10,6 +10,7 @@ import {
 import {
   DIST_BUNDLES, DIST_ROOT, SOURCE_ROOT, PROJECT_ROOT, LICENSE_BANNER, MATERIAL_VERSION
 } from '../constants';
+import {addPureAnnotations} from './annotate-pure';
 
 // There are no type definitions available for these imports.
 const uglify = require('uglify-js');
@@ -38,6 +39,7 @@ export function composeRelease(packageName: string) {
   updatePackageVersion(releasePath);
   createTypingFile(releasePath, packageName);
   createMetadataFile(releasePath, packageName);
+  addPureAnnotationCommentsToEs5Bundle(releasePath, packageName);
 }
 
 /** Builds the bundles for the specified package. */
@@ -156,3 +158,12 @@ function inlinePackageMetadataFiles(packagePath: string) {
     writeFileSync(path , JSON.stringify(metadata), 'utf-8');
   });
 }
+
+/** Adds Uglify "@__PURE__" decorations to the generated ES5 bundle. */
+function addPureAnnotationCommentsToEs5Bundle(outputDir: string, entryName: string) {
+  const es5BundlePath = join(outputDir, '@angular', `${entryName}.es5.js`);
+  const originalContent = readFileSync(es5BundlePath, 'utf-8');
+  const annotatedContent = addPureAnnotations(originalContent);
+
+  writeFileSync(es5BundlePath, annotatedContent, 'utf-8');
+}