use provided AST instead of parsing again

src/Bundle.js

@@ -191,9 +191,17 @@ export default class Bundle {
 				if ( !this.modulePromises[ id ] ) {
 					this.modulePromises[ id ] = Promise.resolve( this.load( id, this.loadOptions ) )
 						.then( source => {
+							let ast;
+
+							if ( typeof source === 'object' ) {
+								ast = source.ast;
+								source = source.code;
+							}
+
 							const module = new Module({
 								id,
 								source,
+								ast,
 								bundle: this
 							});
 

src/Module.js

@@ -29,7 +29,7 @@ function isEmptyExportedVarDeclaration ( node, module, allBundleExports, es6 ) {
 }
 
 export default class Module {
-	constructor ({ id, source, bundle }) {
+	constructor ({ id, source, ast, bundle }) {
 		this.source = source;
 
 		this.bundle = bundle;
@@ -51,7 +51,7 @@ export default class Module {
 		this.suggestedNames = blank();
 		this.comments = [];
 
-		this.statements = this._parse();
+		this.statements = this._parse( ast );
 
 		// imports and exports, indexed by ID
 		this.imports = blank();
@@ -517,11 +517,11 @@ export default class Module {
 	}
 
 	// TODO rename this to parse, once https://github.com/rollup/rollup/issues/42 is fixed
-	_parse () {
+	_parse ( ast ) {
+		// The ast can be supplied programmatically (but usually won't be)
+		if ( !ast ) {
 			// Try to extract a list of top-level statements/declarations. If
 			// the parse fails, attach file info and abort
-		let ast;
-
 			try {
 				ast = parse( this.source, {
 					ecmaVersion: 6,
@@ -533,6 +533,7 @@ export default class Module {
 				err.file = this.id; // see above - not necessarily true, but true enough
 				throw err;
 			}
+		}
 
 		walk( ast, {
 			enter: node => {

test/function/uses-supplied-ast/_config.js

@@ -0,0 +1,35 @@
+var acorn = require( 'acorn' );
+
+var modules = {
+	'main': 'import foo from \'foo\';\nfoo();',
+
+	// the code points to './bar' but the AST points to './baz', so we
+	// can check the AST is being used
+	'foo': {
+		code: 'import bar from \'bar\';\nexport default function foo () {\n\tconsole.log( bar );\n}',
+		ast: acorn.parse( 'import bar from \'baz\';\nexport default function foo () {\n\tconsole.log( bar );\n}', {
+			ecmaVersion: 6,
+			sourceType: 'module'
+		})
+	},
+	
+	'baz': 'export default 42;'
+};
+
+module.exports = {
+	description: 'uses supplied AST',
+	options: {
+		resolveId: function ( importee, importer ) {
+			if ( !importer ) return 'main';
+			return importee;
+		},
+		load: function ( id ) {
+			if ( id === 'bar' ) {
+				throw new Error( 'loaded incorrect module' );
+			}
+
+			return modules[ id ];
+		}
+	},
+	solo: true
+};