Implement breakpoint for closure calls

runtime/vm/debugger_x64.cc

Index: runtime/vm/debugger_x64.cc
===================================================================
--- runtime/vm/debugger_x64.cc	(revision 22595)
+++ runtime/vm/debugger_x64.cc	(working copy)
@@ -30,6 +30,16 @@
 }
 
 
+RawObject* ActivationFrame::GetClosureObject(intptr_t num_actual_args) {
+  // At a minimum we have the closure object on the stack.
+  ASSERT(num_actual_args > 0);
+  // Stack pointer points to last argument that was pushed on the stack.
+  uword closure_addr = sp() + ((num_actual_args - 1) * kWordSize);
+  return reinterpret_cast<RawObject*>(
+             *reinterpret_cast<uword*>(closure_addr));
+}
+
+
 void CodeBreakpoint::PatchFunctionReturn() {
   uint8_t* code = reinterpret_cast<uint8_t*>(pc_ - 13);
   // movq %rbp,%rsp