Dart/Flutter is the least marketable skillet that you could obtain right now in 2018.
If I'm going to take the time to learn a new skillset, I'm going to learn the one that gives me the most marketability and pays the highest that would be iOS/Swift, then Java/Android and then I might play around with Dart/Flutter.
I usually prefer to start with the fundamentals and then learn abstractions/frameworks. Take front end development for example. While frameworks change every six months, knowing the fundamentals of JavaScript/CSS/HTML is valuable long term.
The second sentence doesn't quite make sense to me. Are you saying you hire yourself out to customers as an expert in Java, .Net, C++ & web stack but with only enough detail to produce work that keeps them happy?
Yes, in a way that is it, just I haven't stated that I am an expert, rather that I know them well enough to deliver quality work.
Most companies don't have software as their core business, they just want something that works for their business case, and most of the time they even value more domain knowledge and social skills than beautiful technical solutions.
If it works, fulfils the SLAs and the internal teams are happy with the delivery acceptance review, that is all that matters.
If I'm going to take the time to learn a new skillset, I'm going to learn the one that gives me the most marketability and pays the highest that would be iOS/Swift, then Java/Android and then I might play around with Dart/Flutter.
I usually prefer to start with the fundamentals and then learn abstractions/frameworks. Take front end development for example. While frameworks change every six months, knowing the fundamentals of JavaScript/CSS/HTML is valuable long term.