Updated MallocHooks to collect stack traces when memory is allocated.

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index 7038dd55a292fb5a979a001b752a2e26eaed8f3c..efeb76c81cae39900b3831715a87a93579b6d4f5 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -266,6 +266,12 @@ void OS::DebugBreak() {
 }
 
 
+uintptr_t DART_NOINLINE OS::GetProgramCounter() {
+  return reinterpret_cast<uintptr_t>(
+      __builtin_extract_return_addr(__builtin_return_address(0)));
+}
+
+
 char* OS::StrNDup(const char* s, intptr_t n) {
 // strndup has only been added to Mac OS X in 10.7. We are supplying
 // our own copy here if needed.