This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

More groundwork to get `analyze` and IDE output to agree.

See: https://github.com/dart-lang/sdk/issues/25723

Test that flutter analyze catches no error if two imported libraries
have the same name.

Also, make tests know how to find the flutter root and fix style in one
test to be consistent with the rest of the file.

Removing some special-casing in pursuit of aligning `flutter analyze` output with that provided by server/IDEs.  Specifically:
  * strong-hints (stale, no longer supported)
  * "analyzer says" comment handling in favor of `//ignore`s
  * `conflictingNamePattern` --- no longer needed

Next up some of the error filter regexps...

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

Build bots only have 7.0.

Previously we were ignoring the return code and continuing.

@devoncarew

And fix a zillion omissions this uncovered.

Also, resolve our package conflict, since reflectable has been fixed.

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

This allows building an FLX without leaving any temporary files in the app's
source directory

Fixes #1942.

Most of the remaining widget examples are actually manual tests. This patch
moves them into //dev/manual_tests. A couple are examples of using services,
which I've moved to //examples/layers/services. The remainder are out-dated and
are removed by this patch.

https://github.com/flutter/flutter/issues/2368