Reapply "Allow LetCont to bind multiple continuations."

pkg/compiler/lib/src/cps_ir/redundant_phi.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.
 
 part of dart2js.cps_ir.optimizers;
 
 /// Eliminate redundant phis from the given [FunctionDefinition].
 ///
 /// Phis in this case are [Continuations] together with corresponding
 /// [InvokeContinuation]s. A [Continuation] parameter at position i is redundant
@@ skipping 123 matching lines @@
     }
 
     // Remove trailing items from parameter and argument lists.
     cont.parameters.length = dst;
     for (InvokeContinuation invoke in invokes) {
       invoke.arguments.length = dst;
     }
   }
 
   void processLetCont(LetCont node) {
-    workSet.add(node.continuation);
+    node.continuations.forEach(workSet.add);
   }
 }
 
 /// Returns true, iff [letCont] is not scope of [definition].
 /// Linear in the depth of definition within the IR graph.
 bool _isInScopeOf(LetCont letCont, Definition definition) {
   for (Node node = definition.parent; node != null; node = node.parent) {
     if (node == letCont) {
       return false;
     }
@@ skipping 16 matching lines @@
 
   // Insert it just below the binding of definition.
   InteriorNode binding = definition.parent;
 
   letCont.body = binding.body;
   binding.body.parent = letCont;
 
   binding.body = letCont;
   letCont.parent = binding;
 }