Stabilize label comments (#1179)

Fixes #1160

src/comments.js

@@ -196,6 +196,7 @@ function attach(comments, ast, text, options) {
         handleTemplateLiteralComments(enclosingNode, comment) ||
         handleIfStatementComments(enclosingNode, followingNode, comment) ||
         handleClassComments(enclosingNode, comment) ||
+        handleLabeledStatementComments(enclosingNode, comment) ||
         handleCallExpressionComments(precedingNode, enclosingNode, comment) ||
         handlePropertyComments(enclosingNode, comment) ||
         handleExportNamedDeclarationComments(enclosingNode, comment) ||
@@ -578,6 +579,14 @@ function handleObjectPropertyComments(enclosingNode, comment) {
   return false;
 }
 
+function handleLabeledStatementComments(enclosingNode, comment) {
+  if (enclosingNode && enclosingNode.type === "LabeledStatement") {
+    addLeadingComment(enclosingNode, comment);
+    return true;
+  }
+  return false;
+}
+
 function handleCallExpressionComments(precedingNode, enclosingNode, comment) {
   if (
     enclosingNode &&

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

@@ -1,5 +1,19 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`comment.js 1`] = `
+"{
+  inf_leave: // goto emulation
+  for (;;) {}
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{
+  // goto emulation
+  inf_leave: for (;;) {
+  }
+}
+"
+`;
+
 exports[`empty_label.js 1`] = `
 "a:;
 b

tests/label/comment.js

@@ -0,0 +1,4 @@
+{
+  inf_leave: // goto emulation
+  for (;;) {}
+}