feat: flutter support

[MDemetrio]

*Issue #, if available: #288

Description of changes:
Based on swift implementation ( #255 ), it adds ability to use flutter and flutter-separate transform groups. Examples:
flutter:

//
// StyleDictionary.dart
//
// Do not edit directly
// Generated on Mon, 02 Sep 2019 20:12:07 GMT
//

import 'package:flutter/material.dart';

class StyleDictionary {
  StyleDictionary._();
  
    static const colorBaseGrayDark = Color(0xFF111111);
    static const colorBaseGrayLight = Color(0xFFCCCCCC);
    static const colorBaseGrayMedium = Color(0xFF999999);
    static const colorBaseGreen = Color(0xFF00FF00);
    static const colorBaseRed = Color(0xFFFF0000);
    static const colorFontBase = Color(0xFFFF0000);
    static const colorFontSecondary = Color(0xFF00FF00);
    static const colorFontTertiary = Color(0xFFCCCCCC);
    static const sizeFontBase = 16.00;
    static const sizeFontLarge = 32.00;
    static const sizeFontMedium = 16.00;
    static const sizeFontSmall = 12.00;
}

flutter-separate:

//
// StyleDictionaryColor.dart
//
// Do not edit directly
// Generated on Mon, 02 Sep 2019 20:12:07 GMT
//


import 'package:flutter/material.dart';

class StyleDictionaryColor {
  StyleDictionaryColor._();
  
    static const baseGrayDark = Color(0xFF111111);
    static const baseGrayLight = Color(0xFFCCCCCC);
    static const baseGrayMedium = Color(0xFF999999);
    static const baseGreen = Color(0xFF00FF00);
    static const baseRed = Color(0xFFFF0000);
    static const fontBase = Color(0xFFFF0000);
    static const fontSecondary = Color(0xFF00FF00);
    static const fontTertiary = Color(0xFFCCCCCC);
}

//
// StyleDictionarySize.dart
//
// Do not edit directly
// Generated on Mon, 02 Sep 2019 20:12:07 GMT
//


import 'package:flutter/material.dart';

class StyleDictionarySize {
  StyleDictionarySize._();
  
    static const fontBase = 16.00;
    static const fontLarge = 32.00;
    static const fontMedium = 16.00;
    static const fontSmall = 12.00;
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[chazzmoney]

Thank you for your PR! 👍

It look pretty good to me - just one requested change!

[MDemetrio]

@chazzmoney @dbanksdesign can you guys help me finish this?

[dbanksdesign]

Apologies for taking a while to review this. I left some comments, overall it looks great! Thank you for adding unit tests and adding everything to make a good developer experience. I think the main thing would be moving the example code to its own example in the 'advanced' directory. If that is too much for now, we can add this in without an example and create the example later.

[dbanksdesign]

Looks good! I just have 2 comments/questions. Thank you for your work on this!

[chazzmoney]

This is looking great! Just a couple minor changes to making sure the files are all in good spots and we are good to go!

[chazzmoney]

This is really good! We are very close - no code changes at all.

The reason for the not approved is I'm 100% sure about all files included - some have references to your local machine for example. Can you double check that they should be committed?

Thank you!

[MDemetrio]

@chazzmoney I deleted the files you suggested and tested referencing the package in a application after generating the style-dictionary and flutter files locally, and it all worked fine.

[rodrigosol]

Hi There,

I am looking for flutter support.

How can I help you to finish this PR?

Thanks

[chazzmoney]

LGTM!

[pedropuzzl]

Hello, I'm having problems with color conversion to flutter.

Can you help me? Color tokens are not changing their value after conversion. They are not setting up to Hex8

[umutarpat]

Hello, I want to learn your Roadmap for updating Flutter support with perhaps less bugs or more enchantments.

And do you think right now Flutter support is production ready ?