Preserve new lines after directives (#464)

It's annoying that the line after `"use strict";` gets eaten away on babylon. This fixes it :)

Fixes #216

src/printer.js

@@ -115,6 +115,9 @@ function genericPrintNoParens(path, options, print) {
         path.each(
           function(childPath) {
             parts.push(print(childPath), ";", hardline);
+            if (util.newlineExistsAfter(options.originalText, util.locEnd(childPath.getValue()))) {
+              parts.push(hardline);
+            }
           },
           "directives"
         );

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

@@ -1,3 +1,40 @@
+exports[`test newline.js 1`] = `
+"/* @flow */
+
+\"use strict\";
+
+import a from \"a\";
+
+a();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+/* @flow */
+
+\"use strict\";
+
+import a from \"a\";
+
+a();
+"
+`;
+
+exports[`test newline.js 2`] = `
+"/* @flow */
+
+\"use strict\";
+
+import a from \"a\";
+
+a();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+/* @flow */
+\"use strict\";
+
+import a from \"a\";
+
+a();
+"
+`;
+
 exports[`test test.js 1`] = `
 "\"use strict\";
 
@@ -12,3 +49,18 @@ function fn() {
 }
 "
 `;
+
+exports[`test test.js 2`] = `
+"\"use strict\";
+
+function fn() {
+  \"use strict\";
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+\"use strict\";
+
+function fn() {
+  \"use strict\";
+}
+"
+`;

tests/directives/jsfmt.spec.js

@@ -1 +1,2 @@
 run_spec(__dirname);
+run_spec(__dirname, {parser: 'babylon'});

tests/directives/newline.js

@@ -0,0 +1,7 @@
+/* @flow */
+
+"use strict";
+
+import a from "a";
+
+a();