Switch document generation to use the snippets package instead of the snippets code in the Flutter repo. In the process, some bugs in sample code analysis have been fixed, and I've fixed some more errors in the samples.
This will allow the snippets package to be developed separately from the Flutter repo, and reduce the code in the Flutter repo.
The snippets code is deleted in this PR.
I also converted some comments in the snippet templates to be regular comments instead of doc comments, because having a doc comment block before the imports causes the Dart import sorter to lose the comment. They should have been regular comments in the first place.
The snippets package resides in the assets-for-api-docs repo.
The sample analysis has also been converted to be run in parallel, and I've bumped the Dartdoc version to 1.0.2.
Now that dartdoc automatically generates snapshots for external dart tools, I can remove my path hack from the dartdoc_options.yaml file.
This will allow other packages to again build dartdocs (e.g. plugins) that link to Flutter's dartdocs, and allow me to re-enable dartdoc's cross-linking test that was disabled because of this hack.
When converting all of the samples to use the snippet tool, I encountered some bugs/shortcomings:
1. The document production took 90 minutes, since the snippet tool was being invoked from the command line each time. I fixed this by snapshotting the executable before running, so it's down to 7 minutes.
2. The sample code was not being properly escaped by the snippet tool, so generics were causing issues in the HTML output. It is now quoted.
3. Code examples that used languages other than Dart were not supported. Anything that highlight.js was compiled for dartdoc with is now supported.
4. The comment color for highlight.js was light grey on white, which was pretty unreadable. It's now dark green and bold.
This creates a custom dartdoc tool that will generate snippet blocks in our API docs that allow the user to copy easily to the clipboard, and will also embed the snippet code into a template to show it in a larger context with an app.
This PR adds the snippet tool, a template, and a couple of HTML skeleton files, one for snippets that are designed to be in an application setting, and one where it simply puts a nice container around existing snippets, making them easier to copy to the clipboard.
