Change arrow-function to transfer comments from the old AST (cpojer#50)

For the test, my main concern is that every comment in the input appears
somewhere in the output (even if the order changes).

transforms/__testfixtures__/arrow-function.input.js

@@ -28,6 +28,11 @@ var fn6 = (function() {
   };
 }).bind(this);
 
+var fn7 = /*1*/(/*2*/function/*3*/(/*4*/)/*5*/ {/*6*/
+  console.log('Keep');
+  console.log('comments');
+}/*7*/)/*8*/./*9*/bind/*10*/(/*11*/this/*12*/)/*13*/;
+
 [1, 2, 3].map(function(x) {
   return x * x;
 }.bind(this));
@@ -40,6 +45,10 @@ compare(1, 2, function(num1, num2) {
   return num1 > num2;
 });
 
+/*1*/compare/*2*/(/*3*/1, /*4*/2, /*5*/function/*6*/(/*7*/num1/*8*/, /*9*/num2/*10*/) /*11*/{
+  /*12*/return /*13*/num1 > num2/*14*/;/*15*/
+}/*16*/)/*17*/;/*18*/
+
 Promise.resolve()
 .then(function() {
   console.log('foo');
@@ -48,6 +57,10 @@ Promise.resolve()
   return 4;
 });
 
+foo(function() /*1*/{
+  /*2*/console.log('Keep comments when inlining single expressions');
+/*3*/}/*4*/);
+
 foo(function(a) {
   this.bar(function() {
     return a + this.b;

transforms/__testfixtures__/arrow-function.output.js

@@ -22,18 +22,28 @@ var fn6 = () => ({
   a: 1,
 });
 
+var fn7 = /*2*//*1*/() => /*3*//*4*//*5*/ {/*6*/
+  console.log('Keep');
+  console.log('comments');
+}/*7*//*8*//*10*//*9*//*11*//*12*//*13*/;
+
 [1, 2, 3].map(x => x * x);
 
 [1, 2, 3].map(x => x * x);
 
 compare(1, 2, (num1, num2) => num1 > num2);
 
+/*1*/compare/*2*/(/*3*/1, /*4*/2, /*5*/(/*6*//*7*/num1/*8*/, /*9*/num2/*10*/) => /*13*//*11*//*12*/num1 > num2/*14*//*15*//*16*/)/*17*/;/*18*/
+
 Promise.resolve()
 .then(function() {
   console.log('foo');
 }.bind(this, 'a'))
 .then(a => 4);
 
+foo(() => /*1*//*2*/console.log('Keep comments when inlining single expressions')
+/*3*//*4*/);
+
 foo(function(a) {
   this.bar(function() {
     return a + this.b;

transforms/arrow-function.js

@@ -13,11 +13,13 @@ module.exports = (file, api, options) => {
       fn.body.body.length == 1
     ) {
       const inner = fn.body.body[0];
+      const comments = (fn.body.comments || []).concat(inner.comments || []);
 
       if (
         options['inline-single-expressions'] &&
         inner.type == 'ExpressionStatement'
       ) {
+        inner.expression.comments = (inner.expression.comments || []).concat(comments);
         return inner.expression;
       } else if (inner.type == 'ReturnStatement') {
         const lineStart = fn.loc.start.line;
@@ -29,18 +31,23 @@ module.exports = (file, api, options) => {
         const tooLong = maxWidth && newLength > maxWidth;
 
         if (!tooLong) {
+          inner.argument.comments = (inner.argument.comments || []).concat(comments);
           return inner.argument;
         }
       }
     }
     return fn.body;
   };
 
-  const createArrowFunctionExpression = fn => j.arrowFunctionExpression(
-    fn.params,
-    getBodyStatement(fn),
-    false
-  );
+  const createArrowFunctionExpression = fn => {
+    const arrowFunction = j.arrowFunctionExpression(
+      fn.params,
+      getBodyStatement(fn),
+      false
+    );
+    arrowFunction.comments = fn.comments;
+    return arrowFunction;
+  };
 
   const replacedBoundFunctions = root
     .find(j.CallExpression, {
@@ -61,11 +68,19 @@ module.exports = (file, api, options) => {
       path.value.arguments.length == 1 &&
       path.value.arguments[0].type == 'ThisExpression'
     ))
-    .forEach(path =>
-      j(path).replaceWith(
-        createArrowFunctionExpression(path.value.callee.object)
-      )
-    )
+    .forEach(path => {
+      const comments = path.value.comments || [];
+      for (const node of [path.value.callee, path.value.callee.property, path.value.arguments[0]]) {
+        for (const comment of node.comments || []) {
+          comment.leading = false;
+          comment.trailing = true;
+          comments.push(comment);
+        }
+      }
+      const arrowFunction = createArrowFunctionExpression(path.value.callee.object);
+      arrowFunction.comments = (arrowFunction.comments || []).concat(comments);
+      j(path).replaceWith(arrowFunction);
+    })
     .size() > 0;
 
   const replacedCallbacks = root