This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.
Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.
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.
