Gentle start with removing explicit interfaces

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 11192)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -241,7 +241,7 @@
     __ jmp(is_not_instance_lbl);
     return false;
   }
-  if (type.IsFunctionInterface()) {
+  if (type.IsFunctionType()) {
     // Check if instance is a closure.
     const Immediate raw_null =
         Immediate(reinterpret_cast<intptr_t>(Object::null()));
@@ -254,7 +254,7 @@
   }
   // Custom checking for numbers (Smi, Mint, Bigint and Double).
   // Note that instance is not Smi (checked above).
-  if (type.IsSubtypeOf(Type::Handle(Type::NumberInterface()), NULL)) {
+  if (type.IsSubtypeOf(Type::Handle(Type::Number()), NULL)) {
     GenerateNumberTypeCheck(
         kClassIdReg, type, is_instance_lbl, is_not_instance_lbl);
     return false;
@@ -343,7 +343,7 @@
     __ j(NOT_ZERO, &not_smi, Assembler::kNearJump);
     __ CompareObject(RDI, Type::ZoneHandle(Type::IntInterface()));
     __ j(EQUAL,  is_instance_lbl);
-    __ CompareObject(RDI, Type::ZoneHandle(Type::NumberInterface()));
+    __ CompareObject(RDI, Type::ZoneHandle(Type::Number()));
     __ j(EQUAL,  is_instance_lbl);
     // Smi must be handled in runtime.
     __ jmp(&fall_through);