"Packages get" hangs with package from a SSH based private git repository

## Steps to Reproduce
* Upload a dart package to a private github/gitlab repo
* Set up SSH based authentication
* Add the dependency as:
```
my_package:
git: git@gitlab.com:company/my_package.git
```
### When run in terminal as `flutter packages get`,
It runs fine. When accessing hosted repository, it will ask for passphrase for the private SSH key

### When run using the Packages get button on top left of editor

```
/home/raveesh/MyCode/sdk/flutter/bin/flutter --no-color packages get
Running "flutter packages get" in guru_app...
```
comes up in the messages tab, and the plugin has no way to ask for a SSH password if prompted.
This is where the flutter command hangs and all flutter commands fails because `packages get` does not release the lock

### Current Solution
run:
```
sudo killall dart
```
and then run flutter packages get manually in terminal.

## Version info

```
/home/raveesh/MyCode/sdk/flutter/bin/flutter doctor --verbose
[✓] Flutter (Channel stable, v1.0.0, on Linux, locale en_IN)
• Flutter version 1.0.0 at /home/raveesh/MyCode/sdk/flutter
• Framework revision 5391447fae (6 weeks ago), 2018-11-29 19:41:26 -0800
• Engine revision 7375a0f414
• Dart version 2.1.0 (build 2.1.0-dev.9.4 f9ebf21297)

[✓] Android toolchain - develop for Android devices (Android SDK 28.0.3)
• Android SDK at /home/raveesh/Android/Sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.3
• Java binary at: /home/raveesh/Android/android-studio/jre/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)
• All Android licenses accepted.

[✓] Android Studio (version 3.2)
• Android Studio at /home/raveesh/Android/android-studio
• Flutter plugin version 31.3.1
• Dart plugin version 181.5656
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1136-b06)

[✓] VS Code (version 1.30.2)
• VS Code at /usr/share/code
• Flutter extension version 2.21.1

[✓] Connected device (1 available)
• Moto G 5 Plus • 192.168.1.4:5555 • android-arm • Android 8.1.0 (API 27)

• No issues found!
Process finished with exit code 0
```