Temporary fix to ameliorate severe dart2js regression on some workloads

runtime/vm/jit_optimizer.cc

Index: runtime/vm/jit_optimizer.cc
diff --git a/runtime/vm/jit_optimizer.cc b/runtime/vm/jit_optimizer.cc
index f85d9c46fb3b212ad580ed0ee36a5428469b0e57..c510ff2b50794a66dfcf72f7f00df8be32debb6b 100644
--- a/runtime/vm/jit_optimizer.cc
+++ b/runtime/vm/jit_optimizer.cc
@@ -1543,6 +1543,17 @@ void JitOptimizer::VisitInstanceCall(InstanceCallInstr* instr) {
   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;
   }