Strip no-side-effect imports from Rollup bundles

scripts/rollup/build.js

@@ -408,6 +408,33 @@ function shouldSkipBundle(bundle, bundleType) {
   return false;
 }
 
+// Strip unused require() statements for pure externals modules from the bundle.
+// Rollup's treeshake option should do this, but it doesn't work.
+function stripNoSideEffectImports(bundleType, pureExternalModules, filePath) {
+  const isProduction = isProductionBundleType(bundleType);
+
+  // Dead code elimintation is only applied to production bundles.
+  if (!isProduction) {
+    return;
+  }
+
+  const codeIn = fs.readFileSync(filePath, 'utf-8');
+  let codeOut = codeIn;
+
+  pureExternalModules.forEach(module => {
+    const regExp = new RegExp(
+      `(?<!= *)require\\(["']${module}["']\\)[,;]`,
+      'g'
+    );
+
+    codeOut = codeOut.replace(regExp, '');
+  });
+
+  if (codeIn !== codeOut) {
+    fs.writeFileSync(filePath, codeOut, 'utf-8');
+  }
+}
+
 async function createBundle(bundle, bundleType) {
   if (shouldSkipBundle(bundle, bundleType)) {
     return;
@@ -493,6 +520,9 @@ async function createBundle(bundle, bundleType) {
   try {
     const result = await rollup(rollupConfig);
     await result.write(rollupOutputOptions);
+
+    // HACK to work around the fact that Rollup isn't removing unused, pure-module imports.
+    stripNoSideEffectImports(bundleType, pureExternalModules, mainOutputPath);
   } catch (error) {
     console.log(`${chalk.bgRed.black(' OH NOES! ')} ${logKey}\n`);
     handleRollupError(error);