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

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index 76e976e4666ab1cd9e16afb7a7a254b912776273..2930ea4ad88aa723829e8ac97c1a31fdf673b86a 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -23,7 +23,7 @@ DEFINE_FLAG(bool, trap_on_deoptimization, false, "Trap on deoptimization.");
 DECLARE_FLAG(int, optimization_counter_threshold);
 DECLARE_FLAG(bool, print_ast);
 DECLARE_FLAG(bool, print_scopes);
-DECLARE_FLAG(bool, use_sse41);
+DECLARE_FLAG(bool, eliminate_type_checks);
 
 
 FlowGraphCompiler::~FlowGraphCompiler() {
@@ -561,6 +561,13 @@ void FlowGraphCompiler::GenerateAssertAssignable(intptr_t token_pos,
   __ cmpq(RAX, 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.
+    __ CompareObject(RAX, Object::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());