remove the option to pass in the --no-preview-dart-2 flag to analysis

Enables animated curve diagrams in the documentation, replacing the static images.

Fixes #19617

I changed the wording that it was depending on in my last commit. Oops.

Disallow calling stop() or cancel() multiple times. This means that
when you use startProgress you have to more carefully think about what
exactly is going on.

Properly cancel startProgress in non-ANSI situations, so that
back-to-back startProgress calls all render to the console.

This was added for https://github.com/lemurheavy/coveralls-public/issues/1103 but that didn't help.

This reverts commit c36032c6, because
we're missing some credentials for pushing the gallery.

Submitting TBR to unbreak the build.

TBR=tvolkert

This moves the docs generation and gallery upload to Cirrus CI, which is another step in turning off
Travis.

Fixes #19523

Two clients have run into this in the past so adding a bit of documentation to make it clear. See https://github.com/flutter/flutter/issues/19849 for one example.

72a38a6b1 Build Android engine in presubmit using Cirrus and GCE (#5882)
f9f40badd Log uncaught exceptions from JNI calls at a higher priority (#5891)
723b2784d Update license script README (#5890)
9420da1e2 Roll src/third_party/skia 9acfc6cac2c6..227424b57d28 (10 commits) (#5888)
27c9de59b Roll src/third_party/skia 55b1e6168454..9acfc6cac2c6 (1 commits) (#5887)
8b8c52ed7 Roll src/third_party/skia b935cf8e12d7..55b1e6168454 (1 commits) (#5886)
89602cc64 [scenic] Expose compositor context from rasterizer. (#5739)
9baeb9c1a Roll src/third_party/skia 5f1dc76d0cec..b935cf8e12d7 (1 commits) (#5885)

Fix the gradle templates so that they default to values rather than throw build exceptions. (#19916)

When creating a new project, the build fails with an error similar to:

"versionCode not found. Define flutter.versionCode in the local.properties file."

This puts developers in the untenable situation of having to edit a file with local paths, but being unable to check it in.

This changes the templates to default to using versionCode 1 and version "1.0" if they are not defined in the local.properties file.

Fixes #18983.

This reverts commit e52eeba3.