Add incremental support build by passing previous bundle

src/Bundle.js

@@ -19,6 +19,13 @@ import { dirname, isRelative, relative, resolve } from './utils/path.js';
 
 export default class Bundle {
 	constructor ( options ) {
+		if ( typeof options.bundle === 'object' ) {
+			this.cachedModules = options.bundle.modules.reduce((modules, module) => {
+				modules[module.id] = module;
+				return modules;
+			}, {});
+		}
+
 		this.plugins = ensureArray( options.plugins );
 
 		this.plugins.forEach( plugin => {
@@ -173,7 +180,16 @@ export default class Bundle {
 
 				throw new Error( `Error loading ${id}: load hook should return a string, a { code, map } object, or nothing/null` );
 			})
-			.then( source => transform( source, id, this.transformers ) )
+			.then( source => {
+				if (this.cachedModules && this.cachedModules[id]) {
+					const { code, originalCode } = this.cachedModules[id];
+					return {
+						code,
+						originalCode
+					};
+				}
+				return transform( source, id, this.transformers );
+			})
 			.then( source => {
 				const { code, originalCode, ast, sourceMapChain } = source;
 

src/rollup.js

@@ -27,7 +27,8 @@ const ALLOWED_KEYS = [
 	'plugins',
 	'sourceMap',
 	'treeshake',
-	'useStrict'
+	'useStrict',
+	'bundle'
 ];
 
 export function rollup ( options ) {
@@ -51,9 +52,7 @@ export function rollup ( options ) {
 		return {
 			imports: bundle.externalModules.map( module => module.id ),
 			exports: keys( bundle.entryModule.exports ),
-			modules: bundle.orderedModules.map( module => {
-				return { id: module.id };
-			}),
+			modules: bundle.orderedModules.map( ( { id, code, originalCode } ) => ( { id, code, originalCode } ) ),
 
 			generate: options => bundle.render( options ),
 			write: options => {

test/incremental/expected.js

@@ -0,0 +1,5 @@
+function foo () {
+	console.log('foo');
+};
+
+foo();

test/incremental/foo.js

@@ -0,0 +1,3 @@
+export default function () {
+	console.log('foo');
+};

test/incremental/main.js

@@ -0,0 +1,3 @@
+import foo from './foo.js';
+
+foo();

test/test.js

@@ -13,6 +13,7 @@ var FUNCTION = path.resolve( __dirname, 'function' );
 var FORM = path.resolve( __dirname, 'form' );
 var SOURCEMAPS = path.resolve( __dirname, 'sourcemaps' );
 var CLI = path.resolve( __dirname, 'cli' );
+var INCREMENTAL = path.resolve( __dirname, 'incremental' );
 
 var PROFILES = [
 	{ format: 'amd' },
@@ -423,4 +424,35 @@ describe( 'rollup', function () {
 			});
 		});
 	});
+
+	describe.only('incremental', function () {
+		it('uses previous bundle object to prevent unnecessary transformations', function () {
+			var calls = 0;
+			var counter = {
+				transform: function ( code ) {
+					calls += 1;
+					return code;
+				}
+			};
+			return rollup.rollup({
+				entry: path.join( INCREMENTAL, 'main.js' ),
+				plugins: [counter]
+			}).then( function ( bundle ) {
+				assert.equal( calls, 2 );
+				return rollup.rollup({
+					entry: path.join( INCREMENTAL, 'main.js' ),
+					plugins: [counter],
+					bundle
+				});
+			}).then( function ( bundle ) {
+				assert.equal( calls, 2 );
+				var result = bundle.generate({
+					format: 'es6'
+				});
+				return sander.readFile( path.join( INCREMENTAL, 'expected.js' ) ).then( function (expected) {
+					assert.equal(expected.toString().replace( /(\r\n)/g, '\n'), result.code);
+				});
+			});
+		});
+	});
 });