Prefix core Dart types with 'Dart' when they conflict with built-in .NET types

[JeroMiya]

Lots of the dart code uses Dart types like Dictionary<,>. There are custom subclasses of .net collections with Dart methods added in Globals.cs but in many cases these cause type ambiguity errors in the transpiler output. For example:

https://github.com/JeroMiya/xamarin.flutter/blob/master/FlutterSDK/src/Foundation/Diagnostics.cs#L809

Prefix these conflicting types with Dart, e.g. DartDictionary so that they don't result in ambiguity errors.