[VM/Service] Add await missing from _handleReloadSourcesRequest

This should make reload_sources_with_resident_compiler_test pass
consistently. This change re-enables the test on vm-win-release-x64, and
I will monitor its runs to see if it starts passing consistently.

CoreLibraryReviewExempt: This CL does not include any core library API
changes, only VM Service implementation changes in
sdk/lib/vmservice/running_isolates.dart.
Issue: #59909
Change-Id: I530da624aa926f148ba3ecdffbfc61b0e1048758
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410800
Commit-Queue: Derek Xu <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>

Author: derekxu16

pkg/pkg.status

@@ -69,7 +69,6 @@ vm/testcases/*: SkipByDesign # These are not tests but input for tests.
 vm_service/test/cpu_samples_stream_test: Slow, Pass # Requires CPU sample buffer to fill.
 vm_service/test/get_cpu_samples_rpc_test: Slow, Pass # Does some computation to ensure CPU samples are collected.
 vm_service/test/get_perfetto_cpu_samples_rpc_test: Slow, Pass # Does some computation to ensure CPU samples are collected.
-vm_service/test/reload_sources_with_resident_compiler_test: Skip # issue 59909
 vm_service/test/valid_source_locations_test: ExtraSlow, Pass # Iterates through all libraries and checks for valid source locations.
 vm_snapshot_analysis/test/instruction_sizes_test: Pass, Slow # https://github.com/dart-lang/sdk/issues/60067
 wasm/*: SkipByDesign # These can't be run without running wasm:setup first.
@@ -357,6 +356,7 @@ vm_service/test/eval_with_resident_compiler_test: Skip # issue 59909
 vm_service/test/evaluate_in_frame_rpc_with_resident_compiler_test: Skip # issue 59909
 vm_service/test/evaluate_in_frame_with_scope_with_resident_compiler_test: Skip # issue 59909
 vm_service/test/evaluate_with_scope_with_resident_compiler_test: Skip # issue 59999
+vm_service/test/reload_sources_with_resident_compiler_test: Skip # issue 59909
 
 [ $mode == debug || $runtime != vm || $system == android ]
 vm/test/modular_kernel_plus_aot_test: SkipByDesign # This test should only run if binary is run from build dir

sdk/lib/vmservice/running_isolates.dart

@@ -118,7 +118,7 @@ class RunningIsolates implements MessageRouter {
           message.params[isolate.serviceId];
       reloadKernelRequest.params['kernelFilePath'] =
           outputDill.uri.toFilePath();
-      final response = isolate.routeRequest(service, message);
+      final response = await isolate.routeRequest(service, message);
 
       tempDirectory.deleteSync(recursive: true);
       return response;