Add get dependencies link to pubsec.yaml editor on add/change of a dependency

_From @branflake2267 on July 8, 2017 20:24_

I keep adding dependencies and forgetting to click on get dependencies. I think this should fire automatically, or notify the user to download in the pubsec.yaml like it's done in the dart editor when they have been changed and haven't been downloaded.


This may be redundant, so maybe another fresh notification to the links at top could be used...

## Steps to Reproduce
1. add a dependency
2. don't click on get dependencies
3. goto dart editor, and notice the download dependencies notification at the top of the editor

## Flutter Doctor

```
/Users/branflake2267/git/flutter/bin/flutter --no-color doctor
[✓] Flutter (on Mac OS X 10.12.5 16F73, locale en-US, channel master)
• Flutter at /Users/branflake2267/git/flutter
• Framework revision 4891506931 (21 hours ago), 2017-07-07 16:00:45 -0700
• Engine revision 8ad1576f27
• Tools Dart version 1.25.0-dev.4.0

[✓] Android toolchain - develop for Android devices (Android SDK 26.0.0)
• Android SDK at /Users/branflake2267/Library/Android/sdk
• Platform android-26, build-tools 26.0.0
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_112-release-b06)

[-] iOS toolchain - develop for iOS devices (Xcode 8.3.3)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 8.3.3, Build version 8E3004b
✗ libimobiledevice and ideviceinstaller are not installed or require updating. To update, run:
brew update
brew uninstall --ignore-dependencies libimobiledevice
brew install --HEAD libimobiledevice
brew install ideviceinstaller
• ios-deploy 1.9.1
• CocoaPods version 1.2.1

[✓] Android Studio (version 2.3)
• Android Studio at /Applications/Android Studio.app/Contents
• Java version OpenJDK Runtime Environment (build 1.8.0_112-release-b06)

[✓] IntelliJ IDEA Community Edition (version 2017.1.5)
• Flutter plugin version 15.1
• Dart plugin version 171.4694.29

[✓] IntelliJ IDEA Ultimate Edition (version 2017.1.4)
• Flutter plugin version 14.0
• Dart plugin version 171.4694.29

[✓] Connected devices
• iPhone 7 • 458BC4FE-8205-4369-8718-193B9FC88C8B • ios • iOS 10.3 (simulator)

Process finished with exit code 0
```

_Copied from original issue: flutter/flutter#11123_