[dart] Reject empty output of isolate tool

This mainly fixes test specifications that happened to test a code path
that would result from an error in a real run:
if the step output of the 'isolate archive ...' call is Falsey
(like ''), the shard is run locally.
Some tests for local test runs happened to run this code path by not
providing stdout step data for the isolate call.

This CL adds an error when the isolate hash output is Falsey and
changes the builder 'co19' to not have a fileset and adds another
builder 'co19_sharded' to be used in the tests that actually test
sharding.

Change-Id: Ieb8027d9a7f7f1a386fdaa04be8742adc5068e39
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/195521
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
20 files changed