Reapply "Allow LetCont to bind multiple continuations."

pkg/compiler/lib/src/cps_ir/optimizers.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.cps_ir.optimizers;
 
 import '../constants/expressions.dart' show
     ConstantExpression,
     PrimitiveConstantExpression;
 import '../constants/values.dart';
@@ skipping 33 matching lines @@
   }
 
   void rewriteConstructorDefinition(ConstructorDefinition root) {
     if (root.isAbstract) return;
     rewriteExecutableDefinition(root);
   }
   void rewriteFieldDefinition(FieldDefinition root) {
     if (!root.hasInitializer) return;
     rewriteExecutableDefinition(root);
   }
-}
+}