Fix bad optimization of instance-of with uninstantiated types (issue 5216).

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 12501)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -568,9 +568,10 @@
 
   // Propagated types are not set yet.
   // More checks will possibly be eliminated during type propagation.
-  const bool eliminated = value->CompileTypeIsMoreSpecificThan(dst_type);
-  // Note: 'eliminated' is true for a null constant value, since its type,
-  // i.e. bottom type, is more specific than any type.
+  bool is_null, is_instance;
+  const bool eliminated =
+      (value->CanComputeIsNull(&is_null) && is_null) ||
+      (value->CanComputeIsInstanceOf(dst_type, &is_instance) && is_instance);
   if (FLAG_trace_type_check_elimination) {
     FlowGraphPrinter::PrintTypeCheck(owner()->parsed_function(),
                                      token_pos,