Fix misprinting of directives.

Closes #29.

src/printer.js

@@ -522,7 +522,7 @@ function genericPrintNoParens(path, options, print) {
           parts.push(
             indent(
               options.tabWidth,
-              concat([ line, print(childPath), ";", line ])
+              concat([ hardline, print(childPath), ";", hardline ])
             )
           );
         },

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

@@ -1,3 +1,17 @@
+exports[`test directives.js 1`] = `
+"\"use strict\";
+
+function fn() {
+  \"use strict\";
+}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+\"use strict\";
+
+function fn() {
+  \"use strict\";
+}"
+`;
+
 exports[`test optional-type-name.js 1`] = `
 "type Foo = (any) => string
 

tests/prettier/directives.js

@@ -0,0 +1,5 @@
+"use strict";
+
+function fn() {
+  "use strict";
+}