Can't establish observatory connection when use --local-engine android_debug_unopt . But if I just use "flutter run", everything will be ok.
My step:

1. ./flutter/tools/gn --android --unoptimized
2. ./flutter/tools/gn --unoptimized
3. ninja -C out/android_debug_unopt && ninja -C out/host_debug_unopt
4. flutter create helloworld
5. flutter run -v --local-engine android_debug_unopt

and it stuck at : Waiting for observatory port to be available...

flutter doctor -v
[✓] Flutter (Channel master, v1.1.3-pre.1, on Mac OS X 10.14.3 18D109, locale zh-Hans-CN)
• Flutter version 1.1.3-pre.1 at /Users/mayday/Flutter/flutter
• Framework revision db2ee51035 (3 weeks ago), 2019-03-20 13:53:48 +0800
• Engine revision 3496156
• Dart version 2.2.1 (build 2.2.1-dev.2.0 None)

[!] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /Users/mayday/Android/Development/android_sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• ANDROID_HOME = /Users/mayday/Android/Development/android_sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)
! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[!] iOS toolchain - develop for iOS devices (Xcode 10.2)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 10.2, Build version 10E125
✗ libimobiledevice and ideviceinstaller are not installed. To install with Brew, run:
brew update
brew install --HEAD usbmuxd
brew link usbmuxd
brew install --HEAD libimobiledevice
brew install ideviceinstaller
✗ ios-deploy not installed. To install:
brew install ios-deploy
✗ CocoaPods not installed.
CocoaPods is used to retrieve the iOS platform side's plugin code that responds to your plugin usage on the Dart side.
Without resolving iOS dependencies with CocoaPods, plugins will not work on iOS.
For more info, see https://flutter.io/platform-plugins
To install:
brew install cocoapods
pod setup

[✓] Android Studio (version 3.1)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 29.0.1
• Dart plugin version 173.4700
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b01)

[✓] VS Code (version 1.32.3)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 2.24.0

[✓] Connected device (1 available)
• OS105 • 7389de21 • android-arm64 • Android 7.1.1 (API 25)