This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

* enable lint prefer_asserts_in_initializer_lists

* enable --assert-initializer

This reverts commit c8e4cbf2.

* more widget const constructors

* prefer const constructors

* address review comments

Covers lib/ in package:flutter.

Also, add more dartdoc to widgets.dart.

* Track the worst frame in perf tests

* Use backticks in "/// Returns `null` if"

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Making progress documenting the widget library.

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

Fixes https://github.com/flutter/flutter/issues/1087

 - Adds dartdoc for all the `of` functions.
 - Renames Image to RawImage. This widget is rarely used and shouldn't take up
   such a nice global name.

Fixes #361

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.