[VM/Service] Fix the behaviour of the pause parameter of ReloadSources

The changes to `pkg/vm_service/test/reload_sources_test.dart` in this CL
make it fail if running on a VM that does not also include the changes
to `runtime/vm/service.cc` in this CL. This happens because:

1) `pause_post_service_request` gets carried over from the RPC on line
   44 of `pkg/vm_service/test/reload_sources_test.dart`, and the isolate
   will actually pause after the `getIsolate` RPC within the
   `isolateIsRunning` test on line 51
2) `pause_post_service_request` gets carried over from the RPC on line
   54, and the isolate will actually pause after the `getIsolate` RPC
   within the `isolateIsRunning` test on line 60
3) `isolateIsRunning` on line 64 will fail

TEST=pkg/vm_service/test/reload_sources_test.dart

Change-Id: Id6486c485b1e22599cb920c0874aac0c1aa731c9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397020
Commit-Queue: Derek Xu <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>

Author: derekxu16

pkg/vm_service/test/reload_sources_test.dart

@@ -44,8 +44,11 @@ final tests = <IsolateTest>[
   reloadSources(pause: true),
   // Ensure that we are still stopped at a breakpoint.
   hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_A),
   // Resume the isolate into the while loop.
   resumeIsolate,
+  // Verify that it is running.
+  isolateIsRunning,
   // Reload sources and request to pause post reload. The pause request will
   // be respected because we are not already paused.
   reloadSources(pause: true),

runtime/vm/service.cc

@@ -4002,10 +4002,15 @@ void Service::CheckForPause(Isolate* isolate, JSONStream* stream) {
 }
 
 ErrorPtr Service::MaybePause(Isolate* isolate, const Error& error) {
+  const bool should_pause_post_current_service_request =
+      isolate->should_pause_post_service_request();
+  if (should_pause_post_current_service_request) {
+    // Ensure that we do not accidentally pause post the next service request.
+    isolate->set_should_pause_post_service_request(false);
+  }
   // Don't pause twice.
   if (!isolate->IsPaused()) {
-    if (isolate->should_pause_post_service_request()) {
-      isolate->set_should_pause_post_service_request(false);
+    if (should_pause_post_current_service_request) {
       if (!error.IsNull()) {
         // Before pausing, restore the sticky error. The debugger will return it
         // from PausePostRequest.