Fix broken export comment (#899)

* Add new helper function to convert comments as blocks in ExportNamedDeclaration.

* Add new test.

* Switch to a leading position.

* Update test accordingly.

src/comments.js

@@ -186,7 +186,8 @@ function attach(comments, ast, text, options) {
         handleTemplateLiteralComments(enclosingNode, comment) ||
         handleCallExpressionComments(precedingNode, enclosingNode, comment) ||
         handleClassComments(enclosingNode, comment) ||
-        handlePropertyComments(enclosingNode, comment)
+        handlePropertyComments(enclosingNode, comment) ||
+        handleExportNamedDeclarationComments(enclosingNode, comment)
       ) {
         // We're good
       } else if (precedingNode) {
@@ -575,6 +576,14 @@ function handlePropertyComments(enclosingNode, comment) {
   return false;
 }
 
+function handleExportNamedDeclarationComments(enclosingNode, comment) {
+  if (enclosingNode && enclosingNode.type === "ExportNamedDeclaration") {
+    addLeadingComment(enclosingNode, comment);
+    return true;
+  }
+  return false;
+}
+
 function printComment(commentPath) {
   const comment = commentPath.getValue();
   comment.printed = true;

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

@@ -242,6 +242,24 @@ for (;;);
 "
 `;
 
+exports[`export.js 1`] = `
+"export //comment
+{}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//comment
+export {};
+"
+`;
+
+exports[`export.js 2`] = `
+"export //comment
+{}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//comment
+export {};
+"
+`;
+
 exports[`first-line.js 1`] = `
 "a // comment
 b

tests/comments/export.js

@@ -0,0 +1,2 @@
+export //comment
+{}