Flutter plugin prevents normal dart plugin pub get commands

## Steps to Reproduce

Have a multi module flutter + some library or web based pure dart project.

The dart plugin correctly identifies the pubspec.yaml as a normal dart projects, however clicking the get dependancies (or any other shortcut pub update ) link does not work.

It does work, however, if you set the dart version to be the libexec installed version, and not the sdk that get packaged with flutter.

## Version info

[✓] Flutter (Channel stable, v1.2.1, on Mac OS X 10.14.4 18E227, locale en-US)
• Flutter version 1.2.1 at /Users/drunkengranite/flutter
• Framework revision 8661d8aecd (10 weeks ago), 2019-02-14 19:19:53 -0800
• Engine revision 3757390fa4
• Dart version 2.1.2 (build 2.1.2-dev.0.0 0a7dcf17eb)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /Users/drunkengranite/Library/Android/sdk
• Android NDK location not configured (optional; useful for native profiling
support)
• Platform android-28, build-tools 28.0.3
• Java binary at: /Users/drunkengranite/Library/Application
Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/183.5452501/Android
Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build
1.8.0_152-release-1343-b01)
• All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 10.2.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 10.2.1, Build version 10E1001
• ios-deploy 1.9.4
• CocoaPods version 1.6.1

[✓] Android Studio (version 3.4)
• Android Studio at /Users/drunkengranite/Library/Application
Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/183.5452501/Android
Studio.app/Contents
• Flutter plugin version 34.0.2
• Dart plugin version 183.5901
• Java version OpenJDK Runtime Environment (build
1.8.0_152-release-1343-b01)

[✓] IntelliJ IDEA Ultimate Edition (version 2019.1.1)
• IntelliJ at /Users/drunkengranite/Applications/JetBrains Toolbox/IntelliJ
IDEA Ultimate.app
• Flutter plugin version 34.0.4
• Dart plugin version 191.7019

[✓] VS Code (version 1.33.1)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 2.25.1

[!] Connected device
! No devices available