Specialize breakpoint stubs by set of live registers of the stubs they are intercepting.

runtime/vm/runtime_entry_x64.cc

Index: runtime/vm/runtime_entry_x64.cc
diff --git a/runtime/vm/runtime_entry_x64.cc b/runtime/vm/runtime_entry_x64.cc
index ccd0d6fd3c06d96b6da327d8eb7acb6ebf81697e..143d9001b93aba6fa65ae18800485d171d3488de 100644
--- a/runtime/vm/runtime_entry_x64.cc
+++ b/runtime/vm/runtime_entry_x64.cc
@@ -19,7 +19,7 @@ namespace dart {
 // function. Input for the stub is as follows:
 //   RSP : points to the arguments and return value array.
 //   RBX : address of the runtime function to call.
-//   R10 : number of arguments to the call as Smi.
+//   R10 : number of arguments to the call.
 void RuntimeEntry::Call(Assembler* assembler, intptr_t argument_count) const {
   if (is_leaf()) {
     ASSERT(argument_count == this->argument_count());
@@ -28,8 +28,9 @@ void RuntimeEntry::Call(Assembler* assembler, intptr_t argument_count) const {
   } else {
     // Argument count is not checked here, but in the runtime entry for a more
     // informative error message.
+    ASSERT((GetEntryPoint() & kSmiTagMask) == 0);
     __ movq(RBX, Immediate(GetEntryPoint()));
-    __ movq(R10, Immediate(Smi::RawValue(argument_count)));
+    __ movq(R10, Immediate(argument_count));
     __ Call(&StubCode::CallToRuntimeLabel(), PP);
   }
 }