dart2js cps: Bugfix in redundant phi elimination.

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.
 
 library dart2js.cps_ir.redundant_phi_elimination;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 
 /// Eliminate redundant phis from the given [FunctionDefinition].
@@ skipping 134 matching lines @@
       for (InvokeContinuation invoke in invokes) {
         invoke.arguments[src].unlink();
       }
 
       // Finally, if the substituted definition is not in scope of the affected
       // continuation, move the continuation binding. This is safe to do since
       // the continuation is referenced only as the target in continuation
       // invokes, and all such invokes must be within the scope of
       // [uniqueDefinition]. Note that this is linear in the depth of
       // the binding of [uniqueDefinition].
-      assert(letCont != null);
+      letCont = _makeUniqueBinding(cont);
       _moveIntoScopeOf(letCont, uniqueDefinition);
     }
 
     // Remove trailing items from parameter and argument lists.
     cont.parameters.length = dst;
     for (InvokeContinuation invoke in invokes) {
       invoke.arguments.length = dst;
     }
   }
 
@@ skipping 28 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;
 }
+
+/// Ensures [continuation] has its own LetCont binding by creating
+/// a new LetCont below its current binding, if necessary.
+/// 
+/// Returns the LetCont that now binds [continuation].
+LetCont _makeUniqueBinding(Continuation continuation) {
+  LetCont letCont = continuation.parent;
+  if (letCont.continuations.length == 1) return letCont;
+  letCont.continuations.remove(continuation);
+  LetCont newBinding = new LetCont(continuation, letCont.body);
+  newBinding.body.parent = newBinding;
+  newBinding.parent = letCont;
+  letCont.body = newBinding;
+  continuation.parent = newBinding;
+  return newBinding;
+}