Reapply r18377 it was reverted due to the unrelated bug it surfaced.

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 efb9b7b1d9c88e93f8999f384cbfcf0e15996ace..b39fdbdc8a5d9715ed06a1b758331041154d3ae0 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, eliminate_type_checks);
 
 
 FlowGraphCompiler::~FlowGraphCompiler() {
@@ -561,6 +562,16 @@ void FlowGraphCompiler::GenerateAssertAssignable(intptr_t token_pos,
   __ cmpl(EAX, raw_null);
   __ j(EQUAL, &is_assignable);
 
+  if (!FLAG_eliminate_type_checks) {
+    // If type checks are not eliminated during the graph building then
+    // a transition sentinel can be seen here.
+    const Immediate& raw_transition_sentinel =
+        Immediate(reinterpret_cast<intptr_t>(
+            Object::transition_sentinel().raw()));
+    __ cmpl(EAX, raw_transition_sentinel);
+    __ j(EQUAL, &is_assignable);
+  }
+
   // Generate throw new TypeError() if the type is malformed.
   if (dst_type.IsMalformed()) {
     const Error& error = Error::Handle(dst_type.malformed_error());