Strengthen type assertions post-dominated by checks during type propagation.

runtime/vm/flow_graph_compiler_ia32.cc

Index: runtime/vm/flow_graph_compiler_ia32.cc
diff --git a/runtime/vm/flow_graph_compiler_ia32.cc b/runtime/vm/flow_graph_compiler_ia32.cc
index 72e7d358e1c4525bb064170a9b953232a4a468f3..138ecf33288c78fbf5733f4bc92377f5d9bf7b52 100644
--- a/runtime/vm/flow_graph_compiler_ia32.cc
+++ b/runtime/vm/flow_graph_compiler_ia32.cc
@@ -24,6 +24,7 @@ DEFINE_FLAG(bool, unbox_mints, true, "Optimize 64-bit integer arithmetic.");
 DECLARE_FLAG(int, optimization_counter_threshold);
 DECLARE_FLAG(bool, print_ast);
 DECLARE_FLAG(bool, print_scopes);
+DECLARE_FLAG(bool, enable_type_checks);
 DECLARE_FLAG(bool, eliminate_type_checks);
 
 
@@ -624,6 +625,12 @@ void FlowGraphCompiler::GenerateAssertAssignable(intptr_t token_pos,
 
 void FlowGraphCompiler::EmitInstructionPrologue(Instruction* instr) {
   if (!is_optimizing()) {
+    if (FLAG_enable_type_checks && instr->IsAssertAssignable()) {
+      AssertAssignableInstr* assert = instr->AsAssertAssignable();
+      AddCurrentDescriptor(PcDescriptors::kDeoptBefore,
+                           assert->deopt_id(),
+                           assert->token_pos());
+    }
     AllocateRegistersLocally(instr);
   }
 }