Improve union types with leading comments (#5575)

We're running into this issue at Facebook because `// flowfixme` ignores the very next line, but in this case an empty line is added in-between which breaks the fixme.

Ideally the solution is to avoid adding a newline and we'd call it a day. Unfortunately it is tricky to implement in this particular case. The comment is already printed, including its \n. Yet, we want to indent the block and the only way to convince the doc printer to do so is to add an indent group --before-- the \n is printed, otherwise it's just going to indent on the next \n.

So this PR changes the output from one bad way to another, but the new way has the benefit of not breaking flow for Facebook, which makes our internal teams happier.

Note that the way we print the same construct with `&` is broken in the same way as with this PR. https://prettier.io/playground/#N4Igxg9gdgLgprEAuc0DOMAEBbAngNQEMAbAVzkwF5MAKME4+4xhpl5tmgDgHYA2LgE5+g3gP5CRY6cL6iJE2fL4BKJAB0ombTt17tAegOYAwhGzYEMTftu7oFAGSYYAdwjOYACwBOTzABWACYA1gAeXgBmxCFowVEA5jEBaEGJyV6pIdEBCZkhQQGJIcQpXkGRCSFFWaWRXlWlaOWVwZElCUUlAZgqANwgADQgEAAOMACW6MighD4+EK4ACnMIaMgghABuEBNBQyAARj6EYCFwMADKo6cTUAnIMD7kw14w2MQA6l4T8Gg3YDglzWvwmW1+uA2YDQ62GdzQcB8MCWJwS2EIyEiJARwxSYQAQiczhdLoRLAAZO5wTHYuC4tBhS53JJwACKpAg8BpxBxIBuPgRPg2h0Ih1wxGgB1GPjuME+e28yC4AAZhtKIAjPidRhtpXBBVtqcM-ABHUgTPwowhojFILE8ukgBHYCaPZ6OtDM4hsjlcu204YwUXyoKKpAAJkDJwmxGZZgstpAUAcB1ICIAKqL1v6HQBfXNAA

Fixes #5572

src/language-js/printer-estree.js

@@ -2839,8 +2839,12 @@ function printPathNoParens(path, options, print, args) {
         return join(" | ", printed);
       }
 
+      const shouldAddStartLine =
+        shouldIndent &&
+        !hasLeadingOwnLineComment(options.originalText, n, options);
+
       const code = concat([
-        ifBreak(concat([shouldIndent ? line : "", "| "])),
+        ifBreak(concat([shouldAddStartLine ? line : "", "| "])),
         join(concat([line, "| "]), printed)
       ]);
 

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

@@ -1,5 +1,27 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`comment.js 1`] = `
+====================================options=====================================
+parsers: ["flow", "babylon"]
+printWidth: 80
+                                                                                | printWidth
+=====================================input======================================
+const myValue = (callcallcallcallcallcall(87689769876876897698768768976987687689769876):
+                 // Comment
+                 one | two| thre | jdkxhflksjdhfglkjsdhfglkjhsdkfljghskdjhfgkljshdfgkjhsdkljfhgkljshdfgjdfklgjhklj );
+
+=====================================output=====================================
+const myValue = (callcallcallcallcallcall(
+  87689769876876897698768768976987687689769876
+): // Comment
+| one
+  | two
+  | thre
+  | jdkxhflksjdhfglkjsdhfglkjhsdkfljghskdjhfgkljshdfgkjhsdkljfhgkljshdfgjdfklgjhklj);
+
+================================================================================
+`;
+
 exports[`union.js 1`] = `
 ====================================options=====================================
 parsers: ["flow", "babylon"]

tests/flow_union/comment.js

@@ -0,0 +1,3 @@
+const myValue = (callcallcallcallcallcall(87689769876876897698768768976987687689769876):
+                 // Comment
+                 one | two| thre | jdkxhflksjdhfglkjsdhfglkjhsdkfljghskdjhfgkljshdfgkjhsdkljfhgkljshdfgjdfklgjhklj );