Removes the `null_safety=true` query parameter from DartPad samples in the API docs, since all DartPad channels only support null safety now and the parameter does nothing.
## Test
Removing code, but updates the check in the dartdoc tool for the removal.
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.
This PR modifies the existing API docs samples to use DartPad so that all of the samples are now interactive apps on the API docs site.
It also removes the restriction for the max width of the description area so that the dartpad region can expand horizontally.
I updated the first paragraph on the API docs to indicate that Flutter is more than just mobile now (same text as the README.md at the top level).
I modified a few of the examples so that they looked nicer, and fit better on the page.
I added the sample description text above each DartPad instance, since that often defines the context of the example.
I removed animations and images when they were redundant with the sample content. There were a few that made sense to keep, so I did.
