ass async onwrite hook

src/rollup.js

@@ -1,6 +1,7 @@
 import { basename } from './utils/path.js';
 import { writeFile } from './utils/fs.js';
 import { assign, keys } from './utils/object.js';
+import { mapSequence } from './utils/promise.js';
 import validateKeys from './utils/validateKeys.js';
 import SOURCEMAPPING_URL from './utils/sourceMappingURL.js';
 import Bundle from './Bundle.js';
@@ -96,7 +97,13 @@ export function rollup ( options ) {
 				}
 
 				promises.push( writeFile( dest, code ) );
-				return Promise.all( promises );
+				return Promise.all( promises ).then( () => {
+					return mapSequence( bundle.plugins.filter( plugin => plugin.onwrite ), plugin => {
+						return Promise.resolve( plugin.onwrite( assign({
+							bundle: result
+						}, options )));
+					});
+				});
 			}
 		};
 

test/test.js

@@ -574,5 +574,44 @@ describe( 'rollup', function () {
 				]);
 			});
 		});
+
+		it( 'calls onwrite hooks in sequence', () => {
+			var result = [];
+			var dest = path.join( __dirname, 'tmp/bundle.js' );
+
+			return rollup.rollup({
+				entry: 'entry',
+				plugins: [
+					loader({ entry: `alert('hello')` }),
+					{
+						onwrite ( info ) {
+							return new Promise( ( fulfil ) => {
+								result.push({ a: info.dest, format: info.format });
+								fulfil();
+							});
+						}
+					},
+					{
+						onwrite ( info ) {
+							result.push({ b: info.dest, format: info.format });
+						}
+					}
+				]
+			}).then( bundle => {
+				return bundle.write({
+					dest,
+					format: 'cjs'
+				});
+
+
+			}).then( () => {
+				assert.deepEqual( result, [
+					{ a: dest, format: 'cjs' },
+					{ b: dest, format: 'cjs' }
+				]);
+
+				return sander.unlink( dest );
+			});
+		});
 	});
 });