[vm] Fix bad safepoint transition during AcceptCompilation.

When there is an error, we need to return to the VM state before allocating the exception and its message.

TEST=ci (flaky)
Change-Id: I69f37aee1d3f308ab942f1581fc05501507c5346
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414840
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/vm/isolate_reload.cc

@@ -639,8 +639,11 @@ static intptr_t CommonSuffixLength(const char* a, const char* b) {
 }
 
 static ObjectPtr AcceptCompilation(Thread* thread) {
-  TransitionVMToNative transition(thread);
-  Dart_KernelCompilationResult result = KernelIsolate::AcceptCompilation();
+  Dart_KernelCompilationResult result;
+  {
+    TransitionVMToNative transition(thread);
+    result = KernelIsolate::AcceptCompilation();
+  }
   if (result.status != Dart_KernelCompilationStatus_Ok) {
     if (result.status != Dart_KernelCompilationStatus_MsgFailed) {
       FATAL(