Prepare for dartdoc 0.35.0

[jcollins-g]

Also updates the base analyzer version to 0.40 and the build version to 1.5.0.

Testing with grind compare-sdk-warnings vs 0.34.0 shows performance within margin of error and no difference in warning output.

Testing with grind compare-flutter-warnings shows one more warning, expected due to #2360 (and we can hide when deploying), for Flutter:

  using branch/tag: v0.34.0 for comparison from $DARTDOC_ORIGINAL
*** Flutter repo : 1 new warnings in HEAD, missing in v0.34.0
  warning: package:Flutter has no documentable libraries, from package-Flutter: file:///private/var/folders/b8/jg3_spkj02d51bcz3fsb5jjc000b33/T/fluttergEgkGr/dev/docs
            Dartdoc could not find any public libraries to document in package:Flutter, 
            but documentation was requested.  This might be expected for an 
            asset only package, in which case, disable this warning in your 
            dartdoc_options.yaml file.
*** Flutter repo : Difference in warning output found for 1 warnings (2 warnings found)"

Dartdoc initialization for Flutter is 30% faster, no apparent change to documentation generation performance.

[coveralls]

Coverage increased (+0.04%) to 91.713% when pulling 48f490a on dartdoc-0.35.0 into a0cc2dc on master.

[srawlins]

Yessss love