Provide a way to see inherited values in the Inspector

I'm often faced with one of the following questions:

* Why does this text look like this? (E.g. why is it gray even though I don't tell it to be gray anywhere?)
* Why doesn't this text react to changes in `TextTheme`?
* Where is this text style attribute coming from, exactly?

Text style comes from an inherited widget called `DefaultTextStyle`. In complex apps, there are several layers of `DefaultTextStyle`, and `Text` merges them all into the final style.

In the IntelliJ Flutter inspector (before DevTools integration), it was possible to show the RenderObject/Element tree, and from there, the developer was able to see the `debugLabel`. It was not perfect, but — as far as I know — it was the only way to debug issues like the ones listed above.

DevTools doesn't have a RenderObject/Element tree (https://github.com/flutter/devtools/issues/636), and afaik there is no other way to get to this information.

For what it's worth, I don't think the "go to the Element tree" method was the best developer experience. If we can surface this information better, that would be amazing. Like, in the widget tree above, I can imagine showing information about the `Text`'s current `TextStyle`'s `debugLabel` somewhere in that tree.

まず DevTools Flutter inspector と、RenderObject/Element ツリーが欠落していることに関するリンク先の DevTools issue 636 を確認します。完了条件は、開発者が継承された DefaultTextStyle の値をより明確に調査できるようにすることです。たとえば、ウィジェットツリーに TextStyle の debugLabel を公開します。

索引モデルが issue の本文から書いたものです。