[dds/dap] Try to fix some flaky tests

While reviewing logs for flaky DAP tests, I found that these tests have been failing sometimes with errors that look like the isolate might not have paused on the `debugger()` call yet:

```
evaluateInFrame: (113) Expression compilation error
 _buildExpressionEvaluationScope: invalid 'frameIndex' parameter: 0
```

and

```
evaluateInFrame: (113) Expression compilation error
org-dartlang-debug:synthetic_debug_expression:1:1: Error: Undefined name 'myInstance'.
myInstance
^^^^^^^^^^
```

This waits for the isolate to be paused, not just runnable, which I think might fix this.

Change-Id: Ib1d6b1c44a7ef93452065aa43c4d05058914db28
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395302
Reviewed-by: Derek Xu <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>

Author: DanTup

pkg/dds/test/dap_handler_test.dart

@@ -15,7 +15,7 @@ import 'package:vm_service/vm_service_io.dart';
 
 import 'common/test_helper.dart';
 
-Future<Isolate> waitForFirstRunnableIsolate(VmService service) async {
+Future<Isolate> waitForFirstPausedRunnableIsolate(VmService service) async {
   VM vm;
   do {
     vm = await service.getVM();
@@ -24,7 +24,7 @@ Future<Isolate> waitForFirstRunnableIsolate(VmService service) async {
   Isolate isolate;
   do {
     isolate = await service.getIsolate(isolateId);
-  } while (!isolate.runnable!);
+  } while (!isolate.runnable! || isolate.pauseEvent == null);
   return isolate;
 }
 
@@ -131,7 +131,7 @@ void main() {
 
     // Wait for the isolate to become runnable as `evaluateInFrame` requires
     // the isolate to be runnable.
-    final isolate = await waitForFirstRunnableIsolate(service);
+    final isolate = await waitForFirstPausedRunnableIsolate(service);
     final isolateId = isolate.id!;
 
     // Get the variable for 'myInstance'.
@@ -178,7 +178,7 @@ void main() {
     service = await vmServiceConnectUri(dds!.wsUri!.toString());
 
     // Get the expected isolateId.
-    final isolate = await waitForFirstRunnableIsolate(service);
+    final isolate = await waitForFirstPausedRunnableIsolate(service);
     final isolateId = isolate.id!;
 
     // Ask DAP for all threads.