Do not append input value of AS comparison if the check is eliminated. Fixes issue 4944, crash caus…

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 11916)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -693,10 +693,10 @@
   ASSERT(type.IsFinalized());  // The type in a type cast may be malformed.
   ValueGraphVisitor for_value(owner(), temp_index());
   node->left()->Visit(&for_value);
-  Append(for_value);
   const String& dst_name = String::ZoneHandle(
       Symbols::New(Exceptions::kCastExceptionDstName));
   if (!CanSkipTypeCheck(node->token_pos(), for_value.value(), type, dst_name)) {
+    Append(for_value);
     Do(BuildAssertAssignable(
         node->token_pos(), for_value.value(), type, dst_name));
   }