update to acorn 4

package.json

@@ -47,7 +47,7 @@
   },
   "homepage": "https://github.com/rollup/rollup",
   "devDependencies": {
-    "acorn": "^3.2.0",
+    "acorn": "^4.0.1",
     "buble": "^0.12.5",
     "chalk": "^1.1.3",
     "codecov.io": "^0.1.6",

test/function/double-default-export/_config.js

@@ -1,8 +1,9 @@
+const path = require( 'path' );
 const assert = require( 'assert' );
 
 module.exports = {
 	description: 'throws on double default exports',
 	error: err => {
-		assert.equal( err.message, 'A module can only have one default export' );
+		assert.equal( err.message, `Duplicate export 'default' (2:7) in ${path.resolve(__dirname, 'foo.js')}` );
 	}
 };

test/function/double-named-export/_config.js

@@ -1,8 +1,9 @@
+const path = require( 'path' );
 const assert = require( 'assert' );
 
 module.exports = {
 	description: 'throws on duplicate named exports',
 	error: err => {
-		assert.equal( err.message, `A module cannot have multiple exports with the same name ('foo')` );
+		assert.equal( err.message, `Duplicate export 'foo' (3:9) in ${path.resolve(__dirname, 'foo.js')}` );
 	}
 };

test/function/double-named-reexport/_config.js

@@ -1,8 +1,9 @@
+const path = require( 'path' );
 const assert = require( 'assert' );
 
 module.exports = {
 	description: 'throws on duplicate named exports',
 	error: err => {
-		assert.equal( err.message, `A module cannot have multiple exports with the same name ('foo')` );
+		assert.equal( err.message, `Duplicate export 'foo' (3:9) in ${path.resolve(__dirname, 'foo.js')}` );
 	}
 };