Temporary fix to ameliorate severe dart2js regression on some workloads

runtime/vm/jit_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.
 #ifndef DART_PRECOMPILED_RUNTIME
 #include "vm/jit_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/branch_optimizer.h"
 #include "vm/cha.h"
 #include "vm/compiler.h"
@@ skipping 1525 matching lines @@
   }
 
   if (Token::IsTypeCastOperator(op_kind)) {
     ReplaceWithTypeCast(instr);
     return;
   }
 
   const ICData& unary_checks =
       ICData::ZoneHandle(Z, instr->ic_data()->AsUnaryClassChecks());
 
+  const bool is_dense = CheckClassInstr::IsDenseCidRange(unary_checks);
+  const intptr_t number_of_checks = unary_checks.NumberOfChecks();
+  if (op_kind == Token::kEQ &&
+      number_of_checks > FLAG_max_equality_polymorphic_checks && !is_dense &&
+      flow_graph()->InstanceCallNeedsClassCheck(
+          instr, RawFunction::kRegularFunction)) {
+    // Too many checks, it will be megamorphic which needs unary checks.
+    instr->set_ic_data(&unary_checks);
+    return;
+  }
+
   if ((op_kind == Token::kASSIGN_INDEX) && TryReplaceWithIndexedOp(instr)) {
     return;
   }
   if ((op_kind == Token::kINDEX) && TryReplaceWithIndexedOp(instr)) {
     return;
   }
 
   if (op_kind == Token::kEQ && TryReplaceWithEqualityOp(instr, op_kind)) {
     return;
   }
@@ skipping 314 matching lines @@
   // Discard the environment from the original instruction because the store
   // can't deoptimize.
   instr->RemoveEnvironment();
   ReplaceCall(instr, store);
   return true;
 }
 
 
 }  // namespace dart
 #endif  // DART_PRECOMPILED_RUNTIME