use Rollup

rollup/client.config.js

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+import resolve from 'rollup-plugin-node-resolve';
+import replace from 'rollup-plugin-replace';
+import commonjs from 'rollup-plugin-commonjs';
+import svelte from 'rollup-plugin-svelte';
+import babel from 'rollup-plugin-babel';
+import { terser } from 'rollup-plugin-terser';
+import config from 'sapper/config/rollup.js';
+
+const mode = process.env.NODE_ENV;
+const dev = mode === 'development';
+const legacy = !!process.env.SAPPER_LEGACY_BUILD;
+
+export default {
+	input: config.client.input(),
+	output: config.client.output(),
+	plugins: [
+		svelte({
+			dev,
+			hydratable: true,
+			emitCss: true
+		}),
+		resolve(),
+		replace({
+			'process.browser': true,
+			'process.env.NODE_ENV': JSON.stringify(mode)
+		}),
+		commonjs(),
+
+		legacy && babel({
+			extensions: ['.js', '.html'],
+			runtimeHelpers: true,
+			exclude: ['node_modules/@babel/**'],
+			presets: [
+				['@babel/preset-env', {
+					targets: '> 0.25%, not dead'
+				}]
+			],
+			plugins: [
+				'@babel/plugin-syntax-dynamic-import',
+				['@babel/plugin-transform-runtime', {
+					useESModules: true
+				}]
+			]
+		}),
+
+		!dev && terser({
+			module: true
+		})
+	],
+
+	// temporary, pending Rollup 1.0
+	experimentalCodeSplitting: true,
+
+	// temporary, pending https://github.com/rollup/rollup/pull/2432
+	watch: {
+		chokidar: { disableGlobbing: true }
+	}
+};

rollup/server.config.js

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+import resolve from 'rollup-plugin-node-resolve';
+import replace from 'rollup-plugin-replace';
+import commonjs from 'rollup-plugin-commonjs';
+import svelte from 'rollup-plugin-svelte';
+import config from 'sapper/config/rollup.js';
+import pkg from '../package.json';
+
+const mode = process.env.NODE_ENV;
+const dev = mode === 'development';
+
+export default {
+	input: config.server.input(),
+	output: config.server.output(),
+	plugins: [
+		svelte({
+			generate: 'ssr',
+			dev
+		}),
+		resolve(),
+		replace({
+			'process.browser': true,
+			'process.env.NODE_ENV': JSON.stringify(mode)
+		}),
+		commonjs()
+	],
+	external: Object.keys(pkg.dependencies).concat(
+		require('module').builtinModules
+	),
+
+	// temporary, pending Rollup 1.0
+	experimentalCodeSplitting: true,
+
+	// temporary, pending https://github.com/rollup/rollup/pull/2432
+	watch: {
+		chokidar: { disableGlobbing: true }
+	}
+};

rollup/service-worker.config.js

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+import resolve from 'rollup-plugin-node-resolve';
+import replace from 'rollup-plugin-replace';
+import commonjs from 'rollup-plugin-commonjs';
+import { terser } from 'rollup-plugin-terser';
+import config from 'sapper/config/rollup.js';
+
+const mode = process.env.NODE_ENV;
+const dev = mode === 'development';
+
+export default {
+	input: config.serviceworker.input(),
+	output: config.serviceworker.output(),
+	plugins: [
+		resolve(),
+		replace({
+			'process.browser': true,
+			'process.env.NODE_ENV': JSON.stringify(mode)
+		}),
+		commonjs(),
+		!dev && terser()
+	],
+
+	// temporary, pending https://github.com/rollup/rollup/pull/2432
+	watch: {
+		chokidar: { disableGlobbing: true }
+	}
+};