[code_assets] Organize examples (#2542)

This PR reorganizes the code samples for `package:code_assets`:

* It adds a readme.md to the examples directory to be displayed on pub.dev
* It adds readmes to the samples missing readmes.
* It `.pubignore`s the examples directory except for the readme.
  * And adds a test verifying we're not including samples. (Compressed archive size for code-assets from 3MB -> 21KB.)
* It also adds a test that certain sample files are not moved, since we want to reference them from external sources.

(Also applies some of these things to the other packages.)

Author: dcharkes

pkgs/code_assets/.pubignore

@@ -0,0 +1,19 @@
+# Please keep consistent with .gitignore.
+
+# https://dart.dev/guides/libraries/private-files
+# Created by `dart pub`
+.dart_tool/
+
+# Avoid committing pubspec.lock for library packages; see
+# https://dart.dev/guides/libraries/private-files#pubspeclock.
+pubspec.lock
+
+coverage/
+
+# Additional pub ignores
+doc/
+example/*/*
+test_data/
+test/
+tool/
+

pkgs/code_assets/example/README.md

@@ -1,4 +1,7 @@
-// TODO
+// This file includes parts which are Copyright (c) 1982-1986 Regents
+// of the University of California.  All rights reserved.  The
+// Berkeley Software License Agreement specifies the terms and
+// conditions for redistribution.
 
 // AUTO GENERATED FILE, DO NOT EDIT.
 //