Avoid printing braces for one-statement blocks in js-printer.

pkg/js_ast/lib/src/nodes.dart

Index: pkg/js_ast/lib/src/nodes.dart
diff --git a/pkg/js_ast/lib/src/nodes.dart b/pkg/js_ast/lib/src/nodes.dart
index cc758baabc9913bfa9a06b2042720de8845d0746..b6ce749efe6e522b400fcc8c04d52cba1fc099af 100644
--- a/pkg/js_ast/lib/src/nodes.dart
+++ b/pkg/js_ast/lib/src/nodes.dart
@@ -260,8 +260,8 @@ class EmptyStatement extends Statement {
 
 class If extends Statement {
   final Expression condition;
-  final Node then;
-  final Node otherwise;
+  final Statement then;
+  final Statement otherwise;
 
   If(this.condition, this.then, this.otherwise);
   If.noElse(this.condition, this.then) : this.otherwise = new EmptyStatement();