Fix for issue 27019 (Safepoint assertion failure in Flutter)

runtime/vm/profiler.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/address_sanitizer.h"
 #include "platform/memory_sanitizer.h"
 #include "platform/utils.h"
 
 #include "vm/allocation.h"
 #include "vm/atomic.h"
@@ skipping 931 matching lines @@
 #else
 static uintptr_t __attribute__((noinline)) GetProgramCounter() {
   return reinterpret_cast<uintptr_t>(
       __builtin_extract_return_addr(__builtin_return_address(0)));
 }
 #endif
 
 
 void Profiler::DumpStackTrace(bool native_stack_trace) {
   Thread* thread = Thread::Current();
-  ASSERT(thread != NULL);
+  if (thread == NULL) {
+    return;
+  }
   OSThread* os_thread = thread->os_thread();
   ASSERT(os_thread != NULL);
   Isolate* isolate = thread->isolate();
   if (!CheckIsolate(isolate)) {
     return;
   }
 
   const bool exited_dart_code = thread->HasExitedDartCode();
 
   OS::PrintErr("Dumping %s stack trace for thread %" Px "\n",
@@ skipping 616 matching lines @@
 
 
 ProcessedSampleBuffer::ProcessedSampleBuffer()
     : code_lookup_table_(new CodeLookupTable(Thread::Current())) {
   ASSERT(code_lookup_table_ != NULL);
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart