Work around test failures

[jcollins-g]

This should fix errors like:

https://travis-ci.org/github/dart-lang/dartdoc/jobs/666172974

and:

https://ci.appveyor.com/project/devoncarew/dartdoc/builds/31714590

[jcollins-g]

This assumes that the analyzer is correct at head inferring Map<int, num Function(List<num>)> as the type of importantComputations, below, instead of Map<int, dynamic Function(List<num)> in analyzer 0.39.4. It seems reasonable to me... @stereotype441

/// Type inference mixing with anonymous functions.
final importantComputations = {
  1: (List<num> a) => a[0] + a[1],
  2: (List<num> a) => a[0] - a[1],
  3: (List<num> a) => a[0] * a[1],
  4: (List<num> a) => -a[0]
};

[coveralls]

Coverage remained the same at 92.018% when pulling 34baeb5 on fix-build into f5aaae5 on master.

[stereotype441]

lgtm