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

pkg/compiler/lib/src/js/nodes.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 242 matching lines @@
 class EmptyStatement extends Statement {
   EmptyStatement();
 
   accept(NodeVisitor visitor) => visitor.visitEmptyStatement(this);
   void visitChildren(NodeVisitor visitor) {}
   EmptyStatement _clone() => new EmptyStatement();
 }
 
 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();
 
   bool get hasElse => otherwise is !EmptyStatement;
 
   accept(NodeVisitor visitor) => visitor.visitIf(this);
 
   void visitChildren(NodeVisitor visitor) {
     condition.accept(visitor);
@@ skipping 892 matching lines @@
 class Comment extends Statement {
   final String comment;
 
   Comment(this.comment);
 
   accept(NodeVisitor visitor) => visitor.visitComment(this);
   Comment _clone() => new Comment(comment);
 
   void visitChildren(NodeVisitor visitor) {}
 }