Fix try-catch optimizer.

runtime/vm/redundancy_elimination.cc

 // Copyright (c) 2016, 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.
 
 #include "vm/redundancy_elimination.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph.h"
 #include "vm/hash_map.h"
 #include "vm/il_printer.h"
@@ skipping 3354 matching lines @@
       if (block->try_index() == catch_entry->catch_try_index()) {
         for (ForwardInstructionIterator instr_it(block);
              !instr_it.Done();
              instr_it.Advance()) {
           Instruction* current = instr_it.Current();
           if (current->MayThrow()) {
             Environment* env = current->env()->Outermost();
             ASSERT(env != NULL);
             for (intptr_t env_idx = 0; env_idx < cdefs.length(); ++env_idx) {
               if (cdefs[env_idx] != NULL &&
+                  !cdefs[env_idx]->IsConstant() &&
                   env->ValueAt(env_idx)->BindsToConstant()) {
+                // If the recorded definition is not a constant, record this
+                // definition as the current constant definition.
                 cdefs[env_idx] = env->ValueAt(env_idx)->definition();
               }
               if (cdefs[env_idx] != env->ValueAt(env_idx)->definition()) {
+                // Non-constant definitions are reset to NULL.
                 cdefs[env_idx] = NULL;
               }
             }
           }
         }
       }
     }
     for (intptr_t j = 0; j < idefs->length(); ++j) {
       if (cdefs[j] != NULL && cdefs[j]->IsConstant()) {
         // TODO(fschneider): Use constants from the constant pool.
@@ skipping 93 matching lines @@
         join->phis_ = NULL;
       } else {
         join->phis_->TruncateTo(to_index);
       }
     }
   }
 }
 
 
 }  // namespace dart