The problem was that I copied in a newer change
(flutter/flutter#188473) when I copied in the
localizations stuff. So I got the test changes from that PR, but not the
code change to make the tests pass.

In the future, this PR will come in when we upgrade the SDK for
material_ui and cupertino_ui.

flutter/flutter's flutter_localization package that relates to
material_ui and cupertino_ui. Moves the code to the right place, makes
sure it operates on the correct directories and works correctly and
passes CI, and rewords docs and READMEs to reflect the changes.

 * Creates a *_ui/lib/src/l10n directory.
 * Moves dart classes into material_ui and cupertino_ui.
 * Moves the arb files into material_ui and cupertino_ui.
 * Moves the *_ui l10n tests and makes sure they pass.
 * Moves the localization scripts to script/l10n and updates them to
   work in their new location.
 * Moves READMEs and rewords them, and creates a new README for the
   scripts.
 * Updates the license headers to match flutter/packages.
 * Removed some tests that shouldn't come in until flutter/flutter#188473 is ported to flutter/packages.
 * Skips some tests that failed on web that were previously not run on
   the web at all.

Fixes the tree. `flutter_localizations` contains a patch from flutter/flutter#188473 that has not reached stable yet. This means this test needs to accommodate a different result when tested on stable versus master. Refactored to green the tree. 
@justinmc is filing a follow up issue and looking into extending the code freeze that did not originally apply to localizations.

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