Fix for of/in comment on its own line around in/of (#901)

This is pretty edge case-y so i'm taking the easy way out.

Fixes #886

src/comments.js

@@ -157,6 +157,7 @@ function attach(comments, ast, text, options) {
         handleClassComments(enclosingNode, comment) ||
         handleImportSpecifierComments(enclosingNode, comment) ||
         handleObjectPropertyComments(enclosingNode, comment) ||
+        handleForComments(enclosingNode, precedingNode, comment) ||
         handleUnionTypeComments(
           precedingNode,
           enclosingNode,
@@ -634,6 +635,17 @@ function handleOnlyComments(enclosingNode, ast, comment, isLastComment) {
   return false;
 }
 
+function handleForComments(enclosingNode, precedingNode, comment) {
+  if (enclosingNode && (
+    enclosingNode.type === "ForInStatement" ||
+    enclosingNode.type === "ForOfStatement")
+  ) {
+    addLeadingComment(enclosingNode, comment);
+    return true;
+  }
+  return false;
+}
+
 function printComment(commentPath) {
   const comment = commentPath.getValue();
   comment.printed = true;

tests/for/__snapshots__/jsfmt.spec.js.snap

@@ -1,5 +1,139 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`comment.js 1`] = `
+"for (x
+/*a*/
+in //b
+y) //c
+;
+
+for (x in /*a*/ //b
+y); //c
+
+for (x /*a*/ in y); //b //c
+
+for (x
+//a
+in y);
+
+for(x in
+//a
+y);
+
+for (x
+/*a*/
+of //b
+y) //c
+;
+
+for (x of /*a*/ //b
+y); //c
+
+for (x /*a*/ of y); //b //c
+
+for (x
+//a
+of y);
+
+for(x of
+//a
+y);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+/*a*/
+for (x in y); //b //c
+
+for (x /*a*/ in y); //b //c
+
+for (x /*a*/ in y); //b //c
+
+//a
+for (x in y);
+
+//a
+for (x in y);
+
+/*a*/
+for (x of y); //b //c
+
+for (x /*a*/ of y); //b //c
+
+for (x /*a*/ of y); //b //c
+
+//a
+for (x of y);
+
+//a
+for (x of y);
+"
+`;
+
+exports[`comment.js 2`] = `
+"for (x
+/*a*/
+in //b
+y) //c
+;
+
+for (x in /*a*/ //b
+y); //c
+
+for (x /*a*/ in y); //b //c
+
+for (x
+//a
+in y);
+
+for(x in
+//a
+y);
+
+for (x
+/*a*/
+of //b
+y) //c
+;
+
+for (x of /*a*/ //b
+y); //c
+
+for (x /*a*/ of y); //b //c
+
+for (x
+//a
+of y);
+
+for(x of
+//a
+y);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+/*a*/
+for (x in y); //b //c
+
+for (x /*a*/ in y); //b //c
+
+for (x /*a*/ in y); //b //c
+
+//a
+for (x in y);
+
+//a
+for (x in y);
+
+/*a*/
+for (x of y); //b //c
+
+for (x /*a*/ of y); //b //c
+
+for (x /*a*/ of y); //b //c
+
+//a
+for (x of y);
+
+//a
+for (x of y);
+"
+`;
+
 exports[`for.js 1`] = `
 "for (;;) {}
 for (var i = 0; i < 10; ++i) {}

tests/for/comment.js

@@ -0,0 +1,37 @@
+for (x
+/*a*/
+in //b
+y) //c
+;
+
+for (x in /*a*/ //b
+y); //c
+
+for (x /*a*/ in y); //b //c
+
+for (x
+//a
+in y);
+
+for(x in
+//a
+y);
+
+for (x
+/*a*/
+of //b
+y) //c
+;
+
+for (x of /*a*/ //b
+y); //c
+
+for (x /*a*/ of y); //b //c
+
+for (x
+//a
+of y);
+
+for(x of
+//a
+y);