Ensure directives get printed in block statements (#4873)

packages/babel-generator/src/generators/base.js

@@ -14,11 +14,14 @@ export function Program(node: Object) {
 export function BlockStatement(node: Object) {
   this.token("{");
   this.printInnerComments(node);
-  if (node.body.length) {
+
+  let hasDirectives = node.directives && node.directives.length;
+
+  if (node.body.length || hasDirectives) {
     this.newline();
 
     this.printSequence(node.directives, node, { indent: true });
-    if (node.directives && node.directives.length) this.newline();
+    if (hasDirectives) this.newline();
 
     this.printSequence(node.body, node, { indent: true });
     this.removeTrailingNewline();

packages/babel-generator/test/index.js

@@ -230,6 +230,20 @@ describe("programmatic generation", function() {
     assert.equal(ast.program.body[0].consequent.type, "BlockStatement");
   });
 
+  it("prints directives in block with empty body", function() {
+    let blockStatement = t.blockStatement(
+      [],
+      [t.directive(t.directiveLiteral("use strict"))]
+    );
+
+    let output = generate.default(blockStatement).code;
+    assert.equal(output, [
+      "{",
+      "  \"use strict\";",
+      "}"
+    ].join("\n"));
+  });
+
   it("flow object indentation", function() {
     let objectStatement = t.objectTypeAnnotation(
       [