Track async causal stack traces

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index a5e571f09a5a3e445cc8ff37bed64b3778c5c479..abedeb558d495897de6ac5408face6f2d07c0b45 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -22704,31 +22704,34 @@ static intptr_t PrintOneStackTrace(Zone* zone,
 const char* StackTrace::ToCStringInternal(intptr_t* frame_index,
                                           intptr_t max_frames) const {
   Zone* zone = Thread::Current()->zone();
-  Function& function = Function::Handle();
-  Code& code = Code::Handle();
+  Function& function = Function::Handle(zone);
+  Code& code = Code::Handle(zone);
   // Iterate through the stack frames and create C string description
   // for each frame.
   intptr_t total_len = 0;
   GrowableArray<char*> frame_strings;
   for (intptr_t i = 0; (i < Length()) && (*frame_index < max_frames); i++) {
+    code = CodeAtFrame(i);
     function = FunctionAtFrame(i);
-    if (function.IsNull()) {
+    if (code.raw() == StubCode::AsynchronousGapMarker_entry()->code()) {
+      const char* kAsynchronousGap = "<asynchronous suspension>\n";
+      intptr_t asynchronous_gap_len = strlen(kAsynchronousGap) + 1;
+      frame_strings.Add(const_cast<char*>(kAsynchronousGap));
+      total_len += asynchronous_gap_len;
+    } else if (function.IsNull()) {
       // Check for a null function, which indicates a gap in a StackOverflow or
       // OutOfMemory trace.
       if ((i < (Length() - 1)) &&
           (FunctionAtFrame(i + 1) != Function::null())) {
         const char* kTruncated = "...\n...\n";
         intptr_t truncated_len = strlen(kTruncated) + 1;
-        char* chars = zone->Alloc<char>(truncated_len);
-        OS::SNPrint(chars, truncated_len, "%s", kTruncated);
-        frame_strings.Add(chars);
+        frame_strings.Add(const_cast<char*>(kTruncated));
         total_len += truncated_len;
         ASSERT(PcOffsetAtFrame(i) != Smi::null());
         // To account for gap frames.
         (*frame_index) += Smi::Value(PcOffsetAtFrame(i));
       }
     } else {
-      code = CodeAtFrame(i);
       ASSERT(function.raw() == code.function());
       uword pc = code.PayloadStart() + Smi::Value(PcOffsetAtFrame(i));
       if (code.is_optimized() && expand_inlined() &&