breakpoint_on_if_null_1_test and user_tag_changed_test failing

[bkonyi]

The following two tests are failing after CL 219284 landed:

• pkg/vm_service/test/breakpoint_on_if_null_1_test
• pkg/vm_service/test/user_tag_changed_test

[alexmarkov]

pkg/vm_service/test/breakpoint_on_if_null_4_test also become flaky and start failing on pkg-mac-release-arm64 bot in similar way after 46f9ae9

/==============================================================================================\
| pkg/vm_service/test/breakpoint_on_if_null_4_test broke (Pass -> RuntimeError, expected Pass) |
\==============================================================================================/

--- Command "vm" (took 05.000561s):
DART_CONFIGURATION=ReleaseARM64 CHROME_PATH="/opt/s/w/ir/cache/builder/sdk/third_party/browsers/chrome/Google Chrome.app/Contents/MacOS/Google Chrome" xcodebuild/ReleaseARM64/dart-sdk/bin/dart --enable_asserts --ignore-unrecognized-flags --packages=/opt/s/w/ir/cache/builder/sdk/.packages /opt/s/w/ir/cache/builder/sdk/pkg/vm_service/test/breakpoint_on_if_null_4_test.dart

exit code:
255

stdout:
00:00 �[32m+0�[0m: breakpoint_on_if_null_4_test.dart�[0m

** Launching SERVICE_TEST_TESTEE=true xcodebuild/ReleaseARM64/dart-sdk/bin/dart --disable-dart-dev --pause-isolates-on-start --pause-isolates-on-exit --disable-service-auth-codes --profiler --enable_asserts --ignore-unrecognized-flags --packages=/opt/s/w/ir/cache/builder/sdk/.packages --enable-vm-service:0 /opt/s/w/ir/cache/builder/sdk/pkg/vm_service/test/breakpoint_on_if_null_4_test.dart

** Signaled to run test queries on http://127.0.0.1:51469/

>testee>out> Observatory listening on http://127.0.0.1:51469/
Done loading VM

00:00 �[32m+0�[0m�[31m -1�[0m: breakpoint_on_if_null_4_test.dart �[1m�[31m[E]�[0m�[0m

  streamListen: (103) Stream already subscribed

  package:vm_service/src/vm_service.dart 1719:45      new _OutstandingRequest
  package:vm_service/src/vm_service.dart 2226:21      VmService._call
  package:vm_service/src/vm_service.dart 2173:7       VmService.streamListen
  pkg/vm_service/test/common/test_helper.dart 346:19  _ServiceTesterRunner.getFirstIsolate
  ===== asynchronous gap ===========================
  pkg/vm_service/test/common/test_helper.dart 279:19  _ServiceTesterRunner.run.<fn>.<fn>
  ===== asynchronous gap ===========================
  pkg/vm_service/test/common/test_helper.dart 266:7   _ServiceTesterRunner.run.<fn>
  ===== asynchronous gap ===========================
  dart:async/zone.dart 1281:12                        _CustomZone.bindUnaryCallbackGuarded.<fn>
  

Consider enabling the flag chain-stack-traces to receive more detailed exceptions.
For example, 'dart test --chain-stack-traces'.
All service tests completed successfully.

** Killing script

00:00 �[32m+0�[0m�[31m -1�[0m: �[31mSome tests failed.�[0m

stderr:
Unhandled exception:
Dummy exception to set exit code.

--- Re-run this test:
python3 tools/test.py -n unittest-asserts-release-mac-arm64 pkg/vm_service/test/breakpoint_on_if_null_4_test