Use the new `getIsolatePauseEvent` method from VM service to check for pause event. (#128834)

The `getIsolate` method returns the full list of libraries which can be huge for large apps. Using the more speficic API to only fetch what we need improves hot reload performance.

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

packages/flutter_tools/lib/src/run_hot.dart

@@ -1298,9 +1298,8 @@ Future<ReassembleResult> _defaultReassembleHelper(
     for (final FlutterView view in views) {
       // Check if the isolate is paused, and if so, don't reassemble. Ignore the
       // PostPauseEvent event - the client requesting the pause will resume the app.
-      final vm_service.Isolate? isolate = await device!.vmService!
-        .getIsolateOrNull(view.uiIsolate!.id!);
-      final vm_service.Event? pauseEvent = isolate?.pauseEvent;
+      final vm_service.Event? pauseEvent = await device!.vmService!
+        .getIsolatePauseEventOrNull(view.uiIsolate!.id!);
       if (pauseEvent != null
         && isPauseEvent(pauseEvent.kind!)
         && pauseEvent.kind != vm_service.EventKind.kPausePostRequest) {
@@ -1364,16 +1363,13 @@ Future<ReassembleResult> _defaultReassembleHelper(
       int postReloadPausedIsolatesFound = 0;
       String? serviceEventKind;
       for (final FlutterView view in reassembleViews.keys) {
-        final vm_service.Isolate? isolate = await reassembleViews[view]!
-          .getIsolateOrNull(view.uiIsolate!.id!);
-        if (isolate == null) {
-          continue;
-        }
-        if (isolate.pauseEvent != null && isPauseEvent(isolate.pauseEvent!.kind!)) {
+        final vm_service.Event? pauseEvent = await reassembleViews[view]!
+          .getIsolatePauseEventOrNull(view.uiIsolate!.id!);
+        if (pauseEvent != null && isPauseEvent(pauseEvent.kind!)) {
           postReloadPausedIsolatesFound += 1;
           if (serviceEventKind == null) {
-            serviceEventKind = isolate.pauseEvent!.kind;
-          } else if (serviceEventKind != isolate.pauseEvent!.kind) {
+            serviceEventKind = pauseEvent.kind;
+          } else if (serviceEventKind != pauseEvent.kind) {
             serviceEventKind = ''; // many kinds
           }
         }

packages/flutter_tools/lib/src/vmservice.dart

@@ -1110,6 +1110,22 @@ class FlutterVmService {
         });
   }
 
+  /// Attempt to retrieve the isolate pause event with id [isolateId], or `null` if it has
+  /// been collected.
+  Future<vm_service.Event?> getIsolatePauseEventOrNull(String isolateId) async {
+    return service.getIsolatePauseEvent(isolateId)
+      .then<vm_service.Event?>(
+        (vm_service.Event event) => event,
+        onError: (Object? error, StackTrace stackTrace) {
+          if (error is vm_service.SentinelException ||
+            error == null ||
+            (error is vm_service.RPCError && error.code == RPCErrorCodes.kServiceDisappeared)) {
+            return null;
+          }
+          return Future<vm_service.Event?>.error(error, stackTrace);
+        });
+  }
+
   /// Create a new development file system on the device.
   Future<vm_service.Response> createDevFS(String fsName) {
     // Call the unchecked version of `callServiceExtension` because the caller

packages/flutter_tools/test/general.shard/resident_runner_test.dart

@@ -45,12 +45,19 @@ import '../src/fake_vm_services.dart';
 import '../src/fakes.dart';
 import '../src/testbed.dart';
 
+final vm_service.Event fakeUnpausedEvent = vm_service.Event(
+  kind: vm_service.EventKind.kResume,
+  timestamp: 0
+);
+
+final vm_service.Event fakePausedEvent = vm_service.Event(
+  kind: vm_service.EventKind.kPauseException,
+  timestamp: 0
+);
+
 final vm_service.Isolate fakeUnpausedIsolate = vm_service.Isolate(
   id: '1',
-  pauseEvent: vm_service.Event(
-    kind: vm_service.EventKind.kResume,
-    timestamp: 0
-  ),
+  pauseEvent: fakeUnpausedEvent,
   breakpoints: <vm_service.Breakpoint>[],
   extensionRPCs: <String>[],
   libraries: <vm_service.LibraryRef>[
@@ -72,10 +79,7 @@ final vm_service.Isolate fakeUnpausedIsolate = vm_service.Isolate(
 
 final vm_service.Isolate fakePausedIsolate = vm_service.Isolate(
   id: '1',
-  pauseEvent: vm_service.Event(
-    kind: vm_service.EventKind.kPauseException,
-    timestamp: 0
-  ),
+  pauseEvent: fakePausedEvent,
   breakpoints: <vm_service.Breakpoint>[
     vm_service.Breakpoint(
       breakpointNumber: 123,
@@ -537,11 +541,11 @@ void main() {
       listViews,
       listViews,
       FakeVmServiceRequest(
-        method: 'getIsolate',
+        method: 'getIsolatePauseEvent',
         args: <String, Object>{
           'isolateId': '1',
         },
-        jsonResponse: fakeUnpausedIsolate.toJson(),
+        jsonResponse: fakeUnpausedEvent.toJson(),
       ),
       FakeVmServiceRequest(
         method: 'ext.flutter.reassemble',
@@ -604,11 +608,11 @@ void main() {
         },
       ),
       FakeVmServiceRequest(
-        method: 'getIsolate',
+        method: 'getIsolatePauseEvent',
         args: <String, Object>{
           'isolateId': '1',
         },
-        jsonResponse: fakeUnpausedIsolate.toJson(),
+        jsonResponse: fakeUnpausedEvent.toJson(),
       ),
       FakeVmServiceRequest(
         method: 'ext.flutter.reassemble',
@@ -728,11 +732,11 @@ void main() {
         },
       ),
       FakeVmServiceRequest(
-        method: 'getIsolate',
+        method: 'getIsolatePauseEvent',
         args: <String, Object>{
           'isolateId': '1',
         },
-        jsonResponse: fakeUnpausedIsolate.toJson(),
+        jsonResponse: fakeUnpausedEvent.toJson(),
       ),
       FakeVmServiceRequest(
         method: 'ext.flutter.reassemble',
@@ -791,11 +795,11 @@ void main() {
         },
       ),
       FakeVmServiceRequest(
-        method: 'getIsolate',
+        method: 'getIsolatePauseEvent',
         args: <String, Object>{
           'isolateId': '1',
         },
-        jsonResponse: fakeUnpausedIsolate.toJson(),
+        jsonResponse: fakeUnpausedEvent.toJson(),
       ),
       FakeVmServiceRequest(
         method: 'ext.flutter.fastReassemble',
@@ -882,11 +886,11 @@ void main() {
         },
       ),
       FakeVmServiceRequest(
-        method: 'getIsolate',
+        method: 'getIsolatePauseEvent',
         args: <String, Object>{
           'isolateId': '1',
         },
-        jsonResponse: fakeUnpausedIsolate.toJson(),
+        jsonResponse: fakeUnpausedEvent.toJson(),
       ),
       FakeVmServiceRequest(
         method: 'ext.flutter.fastReassemble',