Fix issue 12336: there was a bug when updating phis at exit of a block with a HTypeKnown instructio…

sdk/lib/_internal/compiler/implementation/ssa/variable_allocator.dart

 // Copyright (c) 2012, 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 ssa;
 
 /**
  * The [LiveRange] class covers a range where an instruction is live.
  */
 class LiveRange {
@@ skipping 664 matching lines @@
       namer.allocateName(instruction);
     }
   }
 
   void handlePhi(HPhi phi, VariableNamer namer) {
     if (!needsName(phi)) return;
 
     for (int i = 0; i < phi.inputs.length; i++) {
       HInstruction input = phi.inputs[i];
       HBasicBlock predecessor = phi.block.predecessors[i];
+      // A [HTypeKnown] instruction never has a name, but its checked
+      // input might, therefore we need to do a copy instead of an
+      // assignment.
+      while (input is HTypeKnown) input = input.inputs[0];
       if (!needsName(input)) {
         names.addAssignment(predecessor, input, phi);
       } else {
         names.addCopy(predecessor, input, phi);
       }
     }
 
     namer.allocateName(phi);
   }
 }