Left sidebar can be overwhelming and doesn't provide sufficient user value

[InMatrix]

Left sidebar on a class page (e.g., AppBar) currently shows every symbols in its library in the alphabetical order. For some large libraries such as material in Flutter, this can be overwhelming to users, as shown in a recent user study.

The team discussed a few potential improvements:

• Collapsing the list by headers (e.g., Classes, CONSTANTS, ENUMS, etc).
• Formalize the "See also" section in many Flutter docs and list the linked pages at the top of the sidebar

• Show N most frequently used classes at the top of the sidebar. We could use Google Analytics data to identify those classes or look into the pub packages corpus.

[Levi-Lesches]

I agree here, it is way too much noise IMO in most libraries. Moreover, when viewing a class, I am way more interested in the members of that class than other classes in the library.

#3464 proposes to replace the left sidebar with the right sidebar -- ie, when viewing a class, the left sidebar lists the class's members. That way, when viewing a constructor/property/method, the sidebar doesn't "jump" from the right to the left.