dart2js: Avoid empty else clauses in generators.

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 "package:compiler/src/js_backend/js_backend.dart" show StringBackedName;
 
 void testTransform(String source, String expected, AsyncRewriterBase rewriter) {
@@ skipping 55 matching lines @@
           // Function start
           closures = [new A.main_closure()];
           __goto = 2;
           return thenHelper(closures, body, __completer);
         case 2:
           // returning from await.
           v0 = __result;
           v1 = 0;
           if (v1 < 0 || v1 >= v0.length)
             H.ioore(v0, v1);
-          else
-            ;
           v2 = 4;
           v3 = 2;
           P.print(v0[v1].call$2(v2, v3));
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   });
@@ skipping 1143 matching lines @@
             // return
             return endOfIteration();
           case 2:
             // rethrow
             return uncaughtError(__currentError);
         }
     };
   });
 }""");
 }