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

tests/compiler/dart2js/async_await_js_transform_test.dart

 // Copyright (c) 2015, 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.
 
 import "package:expect/expect.dart";
 import "package:compiler/src/js/js.dart";
 import "package:compiler/src/js/rewrite_async.dart";
 
 import "backend_dart/dart_printer_test.dart" show PrintDiagnosticListener;
 
@@ skipping 83 matching lines @@
       __outer1:
         switch (__goto) {
           case 0:
             // Function start
             __handler = 3;
           case 7:
             // continue __outer
           case 8:
             // while condition
             __handler = 10;
-            inner: {
+            inner:
               while (true) {
                 __next = [6];
                 // goto finally
                 __goto = 11;
                 break __outer1;
                 break;
               }
-            }
             while (true) {
               __next = [1, 4];
               // goto finally
               __goto = 11;
               break __outer1;
             }
             __goto = 13;
             return thenHelper(foo(), __body, __completer);
           case 13:
             // returning from await.
@@ skipping 854 matching lines @@
           return thenHelper(foobar(x), __body, __completer);
         case 17:
           // returning from await.
           __temp1(__result);
           __returnValue = y;
           // goto return
           __goto = 1;
           break;
         case 14:
           // case
-          if (a) {
+          if (a)
             throw new Error();
-          } else {
+          else {
             // goto while condition
             __goto = 3;
             break;
           }
         case 15:
           // default
           // goto break lab
           __goto = 6;
           break;
         case 9:
@@ skipping 10 matching lines @@
           // return
           return thenHelper(__returnValue, 0, __completer, null);
         case 2:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, __body, __completer, null);
 }""");
 }