Keep the order of execution for require calls

src/index.js

@@ -122,6 +122,10 @@ export default function commonjs ( options = {} ) {
 			const magicString = new MagicString( code );
 
 			let required = {};
+			// Because objects have no guaranteed ordering, yet we need it,
+			// we need to keep track of the order in a array
+			let sources = [];
+
 			let uid = 0;
 
 			let scope = attachScopes( ast, 'scope' );
@@ -202,6 +206,9 @@ export default function commonjs ( options = {} ) {
 					const source = node.arguments[0].value;
 
 					let existing = required[ source ];
+					if ( existing === undefined ) {
+						sources.unshift(source);
+					}
 					let name;
 
 					if ( !existing ) {
@@ -226,8 +233,6 @@ export default function commonjs ( options = {} ) {
 				}
 			});
 
-			const sources = Object.keys( required );
-
 			if ( !sources.length && !uses.module && !uses.exports && !uses.global ) {
 				if ( Object.keys( namedExports ).length ) {
 					throw new Error( `Custom named exports were specified for ${id} but it does not appear to be a CommonJS module` );

test/samples/ordering/bar.js

@@ -0,0 +1,3 @@
+var shared = require('./shared');
+
+module.exports = shared.fooLoaded

test/samples/ordering/foo.js

@@ -0,0 +1,4 @@
+var shared = require('./shared');
+
+// Mutate the shared module
+shared.fooLoaded = true;

test/samples/ordering/main.js

@@ -0,0 +1,5 @@
+require('./foo');
+
+var fooLoaded = require('./bar');
+
+assert.ok( fooLoaded );

test/samples/ordering/shared.js

@@ -0,0 +1,3 @@
+module.exports = {
+	fooLoaded: false
+};

test/test.js

@@ -277,6 +277,13 @@ describe( 'rollup-plugin-commonjs', () => {
 		}).then( executeBundle );
 	});
 
+	it( 'obeys order of require expressions', () => {
+		return rollup({
+			entry: 'samples/ordering/main.js',
+			plugins: [ commonjs() ]
+		}).then( executeBundle );
+	});
+
 	it( 'can ignore references to `global`', () => {
 		return rollup({
 			entry: 'samples/ignore-global/main.js',
@@ -333,7 +340,7 @@ describe( 'rollup-plugin-commonjs', () => {
 			assert.notEqual( module.exports, 'nope' );
 		});
 	});
-	
+
 	it( 'does not process the entry file when it has a leading "." (issue #63)', () => {
 		return rollup({
 			entry: './samples/basic/main.js',