allow the use of double slash flow comments

spec/babel-spec.coffee

@@ -42,6 +42,11 @@ describe "Babel transpiler support", ->
       transpiled = require('./fixtures/babel/flow-comment.js')
       expect(transpiled(3)).toBe 4
 
+  describe 'when a .js file starts with // @flow', ->
+    it "transpiles it using babel", ->
+      transpiled = require('./fixtures/babel/flow-slash-comment.js')
+      expect(transpiled(3)).toBe 4
+
   describe "when a .js file does not start with 'use babel';", ->
     it "does not transpile it using babel", ->
       spyOn(console, 'error')

spec/fixtures/babel/flow-slash-comment.js

@@ -0,0 +1,4 @@
+// @flow
+
+const f: Function = v => v + 1
+module.exports = f

src/babel.js

@@ -11,7 +11,8 @@ var PREFIXES = [
   '/** @babel */',
   '"use babel"',
   '\'use babel\'',
-  '/* @flow */'
+  '/* @flow */',
+  '// @flow'
 ]
 
 var PREFIX_LENGTH = Math.max.apply(Math, PREFIXES.map(function (prefix) {