Specify dart format width in generated files

[turtledev1]

Fixes dart-lang/build#4597

By specifying the width of the generated files, the dart formatter will use that instead of the configured width, therefore not returning an error in pipelines when the config differs.

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[srawlins]

I would rather go the other way and use the configured width for an analysis options file. But it looks like dart_style does not do that automatically with the DartFormatter API, and we'd have to read analysis options ourselves.

This would be something like grabbing the entryLib's path with entryLib.firstFragment.source.fullName and then getting the File for that path with entryLib.session.resourceProvider.getFile and then getting the analysis options for that file with entryLib.session.analysisContext.getAnalysisOptionsForFile and then read the formatterOptions and the pageWidth.

[davidmorgan]

Closing as the dart-lang/mockito repository is merged into the dart-lang/build monorepo. Please re-open this PR there if you'd like to continue to propose it for merging.