Run package:test in --pub-serve mode instead of running pub build

[jakemac53]

See errors here https://chromegw.corp.google.com/i/client.dart.packages/builders/packages-mac-polymer-dart/builds/694/steps/pub%20run%20test/logs/stdio.

Many packages create extra dart files next to the original dart file, and those end up getting ran as separate "tests" when running pub test on the build/test directory. Instead, the bots should run pub serve test ... and then pub run test --pub-serve=8080 ....

[whesse]

Ok, I agree that this is the correct fix to the problems with running 'pub build' and 'pub run test build/test', to change the package bots to use --pub-serve (documentation at https://github.com/dart-lang/test#testing-with-barback).