Improve internal API

src/api/utils/noop.ts

@@ -0,0 +1 @@
+export function noop() {}

src/api/utils/validate_bundler.ts

@@ -0,0 +1,38 @@
+import * as fs from 'fs';
+
+export default function validate_bundler(bundler?: 'rollup' | 'webpack') {
+	if (!bundler) {
+		bundler = (
+			fs.existsSync('rollup.config.js') ? 'rollup' :
+			fs.existsSync('webpack.config.js') ? 'webpack' :
+			null
+		);
+
+		if (!bundler) {
+			// TODO remove in a future version
+			deprecate_dir('rollup');
+			deprecate_dir('webpack');
+
+			throw new Error(`Could not find rollup.config.js or webpack.config.js`);
+		}
+	}
+
+	if (bundler !== 'rollup' && bundler !== 'webpack') {
+		throw new Error(`'${bundler}' is not a valid option for --bundler — must be either 'rollup' or 'webpack'`);
+	}
+
+	return bundler;
+}
+
+function deprecate_dir(bundler: 'rollup' | 'webpack') {
+	try {
+		const stats = fs.statSync(bundler);
+		if (!stats.isDirectory()) return;
+	} catch (err) {
+		// do nothing
+		return;
+	}
+
+	// TODO link to docs, once those docs exist
+	throw new Error(`As of Sapper 0.21, build configuration should be placed in a single ${bundler}.config.js file`);
+}