Add support for parser, compiler as arrow function

index.js

@@ -403,6 +403,7 @@ function unified() {
 function newable(value, name) {
   return (
     typeof value === 'function' &&
+    value.prototype &&
     // A function with keys in its prototype is probably a constructor.
     // Classes’ prototype methods are not enumerable, so we check if some value
     // exists in the prototype.

test/parse.js

@@ -7,7 +7,7 @@ test('parse(file)', function(t) {
   var p = unified()
   var n
 
-  t.plan(12)
+  t.plan(15)
 
   t.throws(
     function() {
@@ -47,6 +47,18 @@ test('parse(file)', function(t) {
     'should return the result `parser` returns if it’s not a constructor'
   )
 
+  p.Parser = (doc, file) => {
+    t.equal(typeof doc, 'string', 'should pass a document')
+    t.ok('message' in file, 'should pass a file')
+    return n
+  }
+
+  t.equal(
+    p.parse('charlie'),
+    n,
+    'should return the result `parser` returns if it’s an arrow function'
+  )
+
   class ESParser {
     constructor(doc, file) {
       t.equal(typeof doc, 'string', 'should pass a document')

test/stringify.js

@@ -10,7 +10,7 @@ test('stringify(node[, file])', function(t) {
   var f
   var n
 
-  t.plan(13)
+  t.plan(16)
 
   t.throws(
     function() {
@@ -51,6 +51,18 @@ test('stringify(node[, file])', function(t) {
     'should return the result `compiler` returns if it’s not a constructor'
   )
 
+  p.Compiler = (node, file) => {
+    t.equal(node, n, 'should pass a node')
+    t.ok('message' in file, 'should pass a file')
+    return 'echo'
+  }
+
+  t.equal(
+    p.stringify(n, f),
+    'echo',
+    'should return the result `compiler` returns if it’s an arrow function'
+  )
+
   p.Compiler = noop.Compiler
 
   t.throws(