Add support for using ES classes to parse, compile

Closes GH-48.
Closes GH-51.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>

index.js

@@ -267,7 +267,7 @@ function unified() {
     Parser = processor.Parser
     assertParser('parse', Parser)
 
-    if (newable(Parser)) {
+    if (newable(Parser, 'parse')) {
       return new Parser(String(file), file).parse()
     }
 
@@ -337,7 +337,7 @@ function unified() {
     assertCompiler('stringify', Compiler)
     assertNode(node)
 
-    if (newable(Compiler)) {
+    if (newable(Compiler, 'compile')) {
       return new Compiler(node, file).compile()
     }
 
@@ -400,8 +400,14 @@ function unified() {
 }
 
 // Check if `value` is a constructor.
-function newable(value) {
-  return typeof value === 'function' && keys(value.prototype)
+function newable(value, name) {
+  return (
+    typeof value === 'function' &&
+    // 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.
+    (keys(value.prototype) || name in value.prototype)
+  )
 }
 
 // Check if `value` is an object with keys.

test/parse.js

@@ -7,7 +7,7 @@ test('parse(file)', function(t) {
   var p = unified()
   var n
 
-  t.plan(8)
+  t.plan(12)
 
   t.throws(
     function() {
@@ -46,4 +46,24 @@ test('parse(file)', function(t) {
     n,
     'should return the result `parser` returns if it’s not a constructor'
   )
+
+  class ESParser {
+    constructor(doc, file) {
+      t.equal(typeof doc, 'string', 'should pass a document')
+      t.ok('message' in file, 'should pass a file')
+    }
+
+    parse() {
+      t.equal(arguments.length, 0, 'should not pass anything to `parse`')
+      return n
+    }
+  }
+
+  p.Parser = ESParser
+
+  t.equal(
+    p.parse('charlie'),
+    n,
+    'should return the result `Parser#parse` returns on an ES class'
+  )
 })

test/stringify.js

@@ -10,7 +10,7 @@ test('stringify(node[, file])', function(t) {
   var f
   var n
 
-  t.plan(9)
+  t.plan(13)
 
   t.throws(
     function() {
@@ -60,4 +60,24 @@ test('stringify(node[, file])', function(t) {
     /Expected node, got `undefined`/,
     'should throw without node'
   )
+
+  class ESCompiler {
+    constructor(node, file) {
+      t.equal(node, n, 'should pass a node')
+      t.ok('message' in file, 'should pass a file')
+    }
+
+    compile() {
+      t.equal(arguments.length, 0, 'should not pass anything to `compile`')
+      return 'echo'
+    }
+  }
+
+  p.Compiler = ESCompiler
+
+  t.equal(
+    p.stringify(n, f),
+    'echo',
+    'should return the result `Compiler#compile` returns on an ES class'
+  )
 })