Repository: flutter
Issue: Current zone changes after 'reassembly'
Labels: framework,dependency: dart,a: debugging,has reproducible steps,P2,found in release: 3.7,found in release: 3.8,team-framework,triaged-framework

## Steps to Reproduce

Current zone changes after "reassembly"

```dart
import 'dart:async';

import 'package:flutter/widgets.dart';

void main() {
  runZoned(
    () {
      runApp(App());
    },
    zoneSpecification: ZoneSpecification(
      print: (Zone self, ZoneDelegate parent, Zone zone, String line) {
        parent.print(zone, 'print in my zone: $line');
      },
    ),
  );
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('Zone.current == ${Zone.current}');
    return const SizedBox();
  }
}
```

**Expected results:**
```dart
print('Zone.current == ${Zone.current}');
```
Output to the console
```
print in my zone: Zone.current == Instance of '_CustomZone'
```

**Actual results:** 

```
print in my zone: Zone.current == Instance of '_CustomZone'
Zone.current == Instance of '_RootZone'
```

<details>
  <summary>Logs</summary>

```
```

```
Analyzing bug...                                                        
No issues found! (ran in 1.3s)

```

```
[✓] Flutter (Channel stable, v1.17.5, on Mac OS X 10.15.6 19G73, locale en-BY)
    • Flutter version 1.17.5 at /Users/nikitadold/development/flutter
    • Framework revision 8af6b2f038 (4 weeks ago), 2020-06-30 12:53:55 -0700
    • Engine revision ee76268252
    • Dart version 2.8.4

 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
    • Android SDK at /Users/nikitadold/Library/Android/sdk
    • Platform android-30, build-tools 30.0.0
    • Java binary at: /Users/nikitadold/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.6, Build version 11E708
    • CocoaPods version 1.9.3

[!] Android Studio (version 4.0)
    • Android Studio at /Users/nikitadold/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/193.6626763/Android Studio.app/Contents
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] IntelliJ IDEA Ultimate Edition (version 2020.1.4)
    • IntelliJ at /Users/nikitadold/Applications/JetBrains Toolbox/IntelliJ IDEA Ultimate.app
    • Flutter plugin version 47.1.3
    • Dart plugin version 201.8538.45

[✓] Connected device (2 available)
    • Pixel 4                   • 99161FFAZ00D3L • android-arm64 • Android 10 (API 29)
    • Android SDK built for x86 • emulator-5554  • android-x86   • Android 10 (API 29) (emulator)

! Doctor found issues in 1 category.
```

</details>