Repository: flutter
Issue: Downloading lines for flutter doctor should display information
Labels: tool,a: quality,P2,team-tool,triaged-tool

## Steps to Reproduce

When `flutter doctor` downloads dependencies, there are no indicators of what is going on. This is particularly important in places with slow Internet connections. I would expect a modern download CLI to include:

* [ ] download size
* [ ] % downloaded
* [ ] progress bar
* [ ] count of how many things will be downloaded (1 of 14...)

## Flutter Doctor

```none
$ flutter doctor
Downloading package sky_engine...(cached)
Downloading common tools...(cached)
Downloading darwin-x64 tools...(cached)
Downloading darwin-x64 tools...
Downloading android-arm-profile/darwin-x64 tools...
Downloading android-arm-release/darwin-x64 tools...
Downloading android-x86 tools...
Downloading android-x64 tools...
Downloading android-arm tools...
Downloading android-arm-profile tools...
Downloading android-arm-release tools...
Downloading ios tools...
Downloading ios-profile tools...
Downloading ios-release tools...
[✓] Flutter (on Mac OS X 10.12.6 16G1114, locale en-US, channel master)
    • Flutter version unknown at /Users/srawlins/code/flutter
    • Framework revision d3705f3ea9 (8 hours ago), 2018-01-24 22:27:24 -0800
    • Engine revision d715860925
    • Tools Dart version 2.0.0-dev.16.0
    • Engine Dart version 2.0.0-edge.8d9d68751a505426eb5f59a9d29f103fde6bd474

[✗] Android toolchain - develop for Android devices
    ✗ Unable to locate Android SDK.
      Install Android Studio from: https://developer.android.com/studio/index.html
      On first launch it will assist you in installing the Android SDK components.
      (or visit https://flutter.io/setup/#android-setup for detailed instructions).
      If Android SDK has been installed to a custom location, set $ANDROID_HOME to that location.

[-] iOS toolchain - develop for iOS devices (Xcode 8.3.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 8.3.3, Build version 8E3004b
    ✗ Flutter requires a minimum Xcode version of 9.0.0.
      Download the latest version or update via the Mac App Store.
    ✗ Your Mac needs to enabled for developer mode before using Xcode for the first time.
      Run 'sudo DevToolsSecurity -enable' to enable developer mode.
    ✗ libimobiledevice and ideviceinstaller are not installed. To install, run:
        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 (not installed)
    • Android Studio not found; download from https://developer.android.com/studio/index.html
      (or visit https://flutter.io/setup/#android-setup for detailed instructions).

[-] Connected devices
    • None
```