Undo workaround for a bug with compile-type of the context value.

runtime/vm/flow_graph_optimizer.cc

 // Copyright (c) 2013, 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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 4563 matching lines @@
           bool inputs_loop_invariant = true;
           for (int i = 0; i < current->InputCount(); ++i) {
             Definition* input_def = current->InputAt(i)->definition();
             if (!input_def->GetBlock()->Dominates(pre_header)) {
               inputs_loop_invariant = false;
               break;
             }
           }
           if (inputs_loop_invariant &&
               !current->IsAssertAssignable() &&
-              !current->IsAssertBoolean() &&
-              !current->IsGuardField()) {
+              !current->IsAssertBoolean()) {
             // TODO(fschneider): Enable hoisting of Assert-instructions
             // if it safe to do.
-            // TODO(15652): Hoisting guard-field instructions causes the
-            // optimizing compiler to crash.
             Hoist(&it, pre_header, current);
           } else if (current->IsCheckSmi() &&
                      current->InputAt(0)->definition()->IsPhi()) {
             TryHoistCheckSmiThroughPhi(
                 &it, header, pre_header, current->AsCheckSmi());
           }
         }
       }
     }
   }
@@ skipping 3721 matching lines @@
   }
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits.length(); i++) {
     CreateMaterializationAt(exits[i], alloc, alloc->cls(), *fields);
   }
 }
 
 
 }  // namespace dart