Fix instance of test for classes that implement call.

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 16535)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -232,8 +232,6 @@
     __ movq(R13, FieldAddress(R13, Class::signature_function_offset()));
     __ cmpq(R13, raw_null);
     __ j(NOT_EQUAL, is_instance_lbl);
-    __ jmp(is_not_instance_lbl);
-    return false;
   }
   // Custom checking for numbers (Smi, Mint, Bigint and Double).
   // Note that instance is not Smi (checked above).