Fix case where async keyword removal breaks syntax.

Fixes #21

index.js

@@ -28,7 +28,7 @@ module.exports = function asyncToGen(source, options) {
 
   // Cheap trick for files that don't actually contain async functions
   if (!(options && options.fastSkip === false) &&
-      source.indexOf('async ') === -1) {
+      source.indexOf('async') === -1) {
     return editor;
   }
 
@@ -293,7 +293,12 @@ function leaveArrowFunction(editor, node, ast) {
 
     var wrapping = createAsyncWrapping(node);
 
-    editor.remove(node.start, node.start + 6);
+    var idx = findTokenIndex(ast.tokens, node.start);
+    while (ast.tokens[idx].value !== 'async') {
+      idx++;
+    }
+    editor.remove(ast.tokens[idx].start, ast.tokens[idx + 1].start);
+
     if (node.body.type === 'BlockStatement') {
       editor.overwrite(node.body.start, node.body.start + 1, wrapping[0]);
       editor.overwrite(node.body.end - 1, node.body.end, wrapping[1]);

test/expected.js

@@ -29,6 +29,9 @@ var arrow1 = () => __async(function*(){
 var arrow2 = () =>__async(function*(){
   return yield 42}());
 
+// async function with minimal whitespace
+var arrow3=()=>return __async(function*(){yield(42)}())
+
 // async obj member function
 var obj = {
   baz() {return __async(function*(){

test/source.js

@@ -29,6 +29,9 @@ var arrow1 = async () => {
 var arrow2 = async () =>
   await 42;
 
+// async function with minimal whitespace
+var arrow3=async()=>await(42)
+
 // async obj member function
 var obj = {
   async baz() {