diff --git a/runtime/vm/gc_sweeper.cc b/runtime/vm/gc_sweeper.cc
index 2b4ee5822201..4cd224377b14 100644
--- a/runtime/vm/gc_sweeper.cc
+++ b/runtime/vm/gc_sweeper.cc
@@ -137,13 +137,14 @@ class SweeperTask : public ThreadPool::Task {
       if (page == last_) break;
       page = next_page;
     }
+    // Exit isolate cleanly *before* notifying it, to avoid shutdown race.
+    Thread::ExitIsolateAsHelper();
     // This sweeper task is done. Notify the original isolate.
     {
       MonitorLocker ml(old_space_->tasks_lock());
       old_space_->set_tasks(old_space_->tasks() - 1);
       ml.Notify();
     }
-    Thread::ExitIsolateAsHelper();
   }
 
  private:
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index b247e566b893..bd19791cf8e7 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1437,6 +1437,15 @@ void Isolate::Shutdown() {
   // avoid exposing it in a state of decay.
   RemoveIsolateFromList(this);
 
+  if (heap_ != NULL) {
+    // Wait for any concurrent GC tasks to finish before shutting down.
+    PageSpace* old_space = heap_->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    while (old_space->tasks() > 0) {
+      ml.Wait();
+    }
+  }
+
   // Create an area where we do have a zone and a handle scope so that we can
   // call VM functions while tearing this isolate down.
   {
@@ -1490,6 +1499,8 @@ void Isolate::Shutdown() {
   // TODO(5411455): For now just make sure there are no current isolates
   // as we are shutting down the isolate.
   Thread::ExitIsolate();
+  // All threads should have exited by now.
+  thread_registry()->CheckNotScheduled(this);
   Profiler::ShutdownProfilingForIsolate(this);
 }
 
diff --git a/runtime/vm/os_thread_android.cc b/runtime/vm/os_thread_android.cc
index 443e800e4d54..ae1b9f2d4e81 100644
--- a/runtime/vm/os_thread_android.cc
+++ b/runtime/vm/os_thread_android.cc
@@ -23,6 +23,14 @@ namespace dart {
   }
 
 
+#if defined(DEBUG)
+#define ASSERT_PTHREAD_SUCCESS(result) VALIDATE_PTHREAD_RESULT(result)
+#else
+// NOTE: This (currently) expands to a no-op.
+#define ASSERT_PTHREAD_SUCCESS(result) ASSERT(result == 0)
+#endif
+
+
 #ifdef DEBUG
 #define RETURN_ON_PTHREAD_FAILURE(result) \
   if (result != 0) { \
@@ -217,7 +225,7 @@ void Mutex::Lock() {
   int result = pthread_mutex_lock(data_.mutex());
   // Specifically check for dead lock to help debugging.
   ASSERT(result != EDEADLK);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -231,7 +239,7 @@ bool Mutex::TryLock() {
   if (result == EBUSY) {
     return false;
   }
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -249,7 +257,7 @@ void Mutex::Unlock() {
   int result = pthread_mutex_unlock(data_.mutex());
   // Specifically check for wrong thread unlocking to aid debugging.
   ASSERT(result != EPERM);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
 }
 
 
diff --git a/runtime/vm/os_thread_linux.cc b/runtime/vm/os_thread_linux.cc
index 5a0fe4fb4fb1..94bb82305451 100644
--- a/runtime/vm/os_thread_linux.cc
+++ b/runtime/vm/os_thread_linux.cc
@@ -24,6 +24,14 @@ namespace dart {
   }
 
 
+#if defined(DEBUG)
+#define ASSERT_PTHREAD_SUCCESS(result) VALIDATE_PTHREAD_RESULT(result)
+#else
+// NOTE: This (currently) expands to a no-op.
+#define ASSERT_PTHREAD_SUCCESS(result) ASSERT(result == 0)
+#endif
+
+
 #ifdef DEBUG
 #define RETURN_ON_PTHREAD_FAILURE(result) \
   if (result != 0) { \
@@ -218,7 +226,7 @@ void Mutex::Lock() {
   int result = pthread_mutex_lock(data_.mutex());
   // Specifically check for dead lock to help debugging.
   ASSERT(result != EDEADLK);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -232,7 +240,7 @@ bool Mutex::TryLock() {
   if (result == EBUSY) {
     return false;
   }
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -250,7 +258,7 @@ void Mutex::Unlock() {
   int result = pthread_mutex_unlock(data_.mutex());
   // Specifically check for wrong thread unlocking to aid debugging.
   ASSERT(result != EPERM);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
 }
 
 
diff --git a/runtime/vm/os_thread_macos.cc b/runtime/vm/os_thread_macos.cc
index 2b870db479d3..6a9aca22cd58 100644
--- a/runtime/vm/os_thread_macos.cc
+++ b/runtime/vm/os_thread_macos.cc
@@ -31,6 +31,14 @@ namespace dart {
   }
 
 
+#if defined(DEBUG)
+#define ASSERT_PTHREAD_SUCCESS(result) VALIDATE_PTHREAD_RESULT(result)
+#else
+// NOTE: This (currently) expands to a no-op.
+#define ASSERT_PTHREAD_SUCCESS(result) ASSERT(result == 0)
+#endif
+
+
 #ifdef DEBUG
 #define RETURN_ON_PTHREAD_FAILURE(result) \
   if (result != 0) { \
@@ -223,7 +231,7 @@ void Mutex::Lock() {
   int result = pthread_mutex_lock(data_.mutex());
   // Specifically check for dead lock to help debugging.
   ASSERT(result != EDEADLK);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -237,7 +245,7 @@ bool Mutex::TryLock() {
   if ((result == EBUSY) || (result == EDEADLK)) {
     return false;
   }
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
   // When running with assertions enabled we do track the owner.
 #if defined(DEBUG)
   owner_ = OSThread::GetCurrentThreadId();
@@ -255,7 +263,7 @@ void Mutex::Unlock() {
   int result = pthread_mutex_unlock(data_.mutex());
   // Specifically check for wrong thread unlocking to aid debugging.
   ASSERT(result != EPERM);
-  ASSERT(result == 0);  // Verify no other errors.
+  ASSERT_PTHREAD_SUCCESS(result);  // Verify no other errors.
 }
 
 
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index a82c8b3a358d..bbdd1b401859 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -27,6 +27,12 @@ static void DeleteThread(void* thread) {
 }
 
 
+Thread::~Thread() {
+  // We should cleanly exit any isolate before destruction.
+  ASSERT(isolate_ == NULL);
+}
+
+
 void Thread::InitOnceBeforeIsolate() {
   ASSERT(thread_key_ == OSThread::kUnsetThreadLocalKey);
   thread_key_ = OSThread::CreateThreadLocal(DeleteThread);
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 00cea34611a3..bc9cb61dcd69 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -75,6 +75,8 @@ class Thread {
   static void InitOnceBeforeIsolate();
   static void InitOnceAfterObjectAndStubCode();
 
+  ~Thread();
+
   // The topmost zone used for allocation in this thread.
   Zone* zone() const { return state_.zone; }
 
diff --git a/runtime/vm/thread_registry.h b/runtime/vm/thread_registry.h
index d4de8a3fa469..af160269fa33 100644
--- a/runtime/vm/thread_registry.h
+++ b/runtime/vm/thread_registry.h
@@ -63,6 +63,19 @@ class ThreadRegistry {
     return (FindEntry(thread) != NULL);
   }
 
+  void CheckNotScheduled(Isolate* isolate) {
+    MutexLocker ml(mutex_);
+    for (int i = 0; i < entries_.length(); ++i) {
+      const Entry& entry = entries_[i];
+      if (entry.scheduled) {
+        FATAL3("Isolate %p still scheduled on %p (whose isolate_ is %p)\n",
+               isolate,
+               entry.thread,
+               entry.thread->isolate());
+      }
+    }
+  }
+
   void VisitObjectPointers(ObjectPointerVisitor* visitor) {
     MutexLocker ml(mutex_);
     for (int i = 0; i < entries_.length(); ++i) {