This reverts commit c64ace84.

* Revert "Revert "Enable developers to run pod from terminal directly and skip pod install if possible. (#13374)" (#13770)"

This reverts commit 0759043e.

* some nits on cocoapods code

* put back the FLUTTER_FRAMEWORK_DIR env variable

Revert "Enable developers to run pod from terminal directly and skip pod install if possible. (#13374)" (#13770)

This reverts commit c6a17525.

Avoids running pod install if .flutter-plugins and its flutter framework pod dependency didn't change

* start

* with create

* refactor cocoapod code, add tests

* fix tests

* throw when cocoapod missing

* obj-c projects don’t use use_framework!

This reverts commit 4bbf158b.

This yak shave went as follows:

Fix https://github.com/flutter/flutter/issues/8795 by adding stocks to
the examples README.

Notice the layers entry in that README isn't quite right either.
Update that.

Check the layers/README file is worth pointing at.

Update the layers/README.

Let's run some of the layer tests to see if they still work.

Oops, need to update them to gradle.

Ok let's try running them again.

Oops, sector is broken.

Add a test for sector.

Fix sector. Find you need to add an assert to a const constructor.

Notice we need to turn const asserts on for the analyzer.

Notice the analysis_options files are out of sync with each other and
with the full list of lints.

Turn on the lints that should be on.

Fix the bugs that finds.

Go through all packages brought in by pub, and write the name and path of every one that is a flutter plugin into .flutter-plugins.

In android/settings.gradle and ios/Podfile, read in .flutter-plugins, if that file exists. The Android / iOS code from the plugins is automatically added as dependencies of the native code of the app.