make use strict optional

src/finalisers/amd.js

@@ -15,7 +15,8 @@ export default function amd ( bundle, magicString, { exportMode, indentString },
 		( options.moduleId ? `'${options.moduleId}', ` : `` ) +
 		( deps.length ? `[${deps.join( ', ' )}], ` : `` );
 
-	const intro = `define(${params}function (${args.join( ', ' )}) { 'use strict';\n\n`;
+	const useStrict = options.useStrict !== false ? ` 'use strict';` : ``;
+	const intro = `define(${params}function (${args.join( ', ' )}) {${useStrict}\n\n`;
 
 	// var foo__default = 'default' in foo ? foo['default'] : foo;
 	const interopBlock = getInteropBlock( bundle );

src/finalisers/cjs.js

@@ -1,7 +1,7 @@
 import getExportBlock from './shared/getExportBlock';
 
-export default function cjs ( bundle, magicString, { exportMode }) {
-	let intro = `'use strict';\n\n`;
+export default function cjs ( bundle, magicString, { exportMode }, options ) {
+	let intro = options.useStrict === false ? `` : `'use strict';\n\n`;
 
 	// TODO handle empty imports, once they're supported
 	const importBlock = bundle.externalModules

src/finalisers/iife.js

@@ -21,7 +21,8 @@ export default function iife ( bundle, magicString, { exportMode, indentString }
 		args.unshift( 'exports' );
 	}
 
-	let intro = `(function (${args}) { 'use strict';\n\n`;
+	const useStrict = options.useStrict !== false ? ` 'use strict';` : ``;
+	let intro = `(function (${args}) {${useStrict}\n\n`;
 	let outro = `\n\n})(${dependencies});`;
 
 	if ( exportMode === 'default' ) {

src/finalisers/umd.js

@@ -33,12 +33,14 @@ export default function umd ( bundle, magicString, { exportMode, indentString },
 	const cjsExport = exportMode === 'default' ? `module.exports = ` : ``;
 	const defaultExport = exportMode === 'default' ? `global.${options.moduleName} = ` : '';
 
+	const useStrict = options.useStrict !== false ? ` 'use strict';` : ``;
+
 	const intro =
 		`(function (global, factory) {
 			typeof exports === 'object' && typeof module !== 'undefined' ? ${cjsExport}factory(${cjsDeps.join( ', ' )}) :
 			typeof define === 'function' && define.amd ? define(${amdParams}factory) :
 			${defaultExport}factory(${globalDeps});
-		}(this, function (${args}) { 'use strict';
+		}(this, function (${args}) {${useStrict}
 
 		`.replace( /^\t\t/gm, '' ).replace( /^\t/gm, magicString.getIndentString() );