Fix crash in code coverage generation.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 16b70e466c5ecd6054680bbd4b05c90e9340ea08..8efe59435d5eef4f5cb150033e73f3c0393d2557 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1433,12 +1433,23 @@ void Isolate::Shutdown() {
   }
 #endif  // DEBUG
 
+  // First, perform higher-level cleanup that may need to allocate.
+  {
+    // Ensure we have a zone and handle scope so that we can call VM functions.
+    StackZone stack_zone(this);
+    HandleScope handle_scope(this);
+
+    // Write out the coverage data if collection has been enabled.
+    CodeCoverage::Write(this);
+  }
+
   // Remove this isolate from the list *before* we start tearing it down, to
   // avoid exposing it in a state of decay.
   RemoveIsolateFromList(this);
 
   if (heap_ != NULL) {
     // Wait for any concurrent GC tasks to finish before shutting down.
+    // TODO(koda): Support faster sweeper shutdown (e.g., after current page).
     PageSpace* old_space = heap_->old_space();
     MonitorLocker ml(old_space->tasks_lock());
     while (old_space->tasks() > 0) {
@@ -1446,11 +1457,13 @@ void Isolate::Shutdown() {
     }
   }
 
-  // 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.
+  // Then, proceed with low-level teardown.
   {
+    // Ensure we have a zone and handle scope so that we can call VM functions,
+    // but we no longer allocate new heap objects.
     StackZone stack_zone(this);
     HandleScope handle_scope(this);
+    NoSafepointScope no_safepoint_scope;
 
     if (compiler_stats_ != NULL) {
       compiler_stats()->Print();
@@ -1474,9 +1487,6 @@ void Isolate::Shutdown() {
     // Dump all accumulated timer data for the isolate.
     timer_list_.ReportTimers();
 
-    // Write out the coverage data if collection has been enabled.
-    CodeCoverage::Write(this);
-
     // Finalize any weak persistent handles with a non-null referent.
     FinalizeWeakPersistentHandlesVisitor visitor;
     api_state()->weak_persistent_handles().VisitHandles(&visitor);
@@ -1496,6 +1506,15 @@ void Isolate::Shutdown() {
     }
   }
 
+#if defined(DEBUG)
+  // No concurrent sweeper tasks should be running at this point.
+  if (heap_ != NULL) {
+    PageSpace* old_space = heap_->old_space();
+    MonitorLocker ml(old_space->tasks_lock());
+    ASSERT(old_space->tasks() == 0);
+  }
+#endif
+
   // TODO(5411455): For now just make sure there are no current isolates
   // as we are shutting down the isolate.
   Thread::ExitIsolate();