Ensure there is a line break after leading JSX white space (#2348)

src/printer.js

@@ -3938,8 +3938,13 @@ function printJSXElement(path, options, print) {
     // whitespace as `{" "}` when outputting this element over multiple lines.
     if (child === jsxWhitespace) {
       if (i === 1 && children[i - 1] === "") {
+        if (children.length === 2) {
+          // Solitary whitespace
+          multilineChildren.push(rawJsxWhitespace);
+          return;
+        }
         // Leading whitespace
-        multilineChildren.push(rawJsxWhitespace);
+        multilineChildren.push(concat([rawJsxWhitespace, hardline]));
         return;
       } else if (i === children.length - 1) {
         // Trailing whitespace

tests/flow/more_react/__snapshots__/jsfmt.spec.js.snap

@@ -164,7 +164,8 @@ var App = require("App.react");
 
 var app = (
   <App y={42}>
-    {" "}// error, y: number but foo expects string in App.react Some text.
+    {" "}
+    // error, y: number but foo expects string in App.react Some text.
   </App>
 );
 

tests/jsx-significant-space/__snapshots__/jsfmt.spec.js.snap

@@ -171,7 +171,8 @@ nest_plz = (
 
 regression_not_transformed_1 = (
   <span>
-    {" "}<Icon icon="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
+    {" "}
+    <Icon icon="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
   </span>
 );
 
@@ -183,7 +184,8 @@ regression_not_transformed_2 = (
 
 similar_1 = (
   <span>
-    {" "}<Icon icon="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
+    {" "}
+    <Icon icon="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" />
   </span>
 );
 

tests/jsx-stateless-arrow-fn/__snapshots__/jsfmt.spec.js.snap

@@ -150,7 +150,8 @@ const render7A = () =>
 const render7B = () =>
   <div>
     <span>
-      {" "}<span /> Dont break plz
+      {" "}
+      <span /> Dont break plz
     </span>
     <span>
       <span />Dont break plz

tests/jsx-text-wrap/__snapshots__/jsfmt.spec.js.snap

@@ -432,8 +432,9 @@ x = (
 
 leading_whitespace = (
   <div>
-    {" "}First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth
-    Eleventh Twelfth Thirteenth Fourteenth
+    {" "}
+    First Second Third Fourth Fifth Sixth Seventh Eighth Ninth Tenth Eleventh
+    Twelfth Thirteenth Fourteenth
   </div>
 );