pub run should not emit any output not from the executable

[jathak]

Environment

• pub version or flutter pub version: Pub 2.9.0-19.0-dev
• OS version: CI on Ubuntu, macOS, and Windows, locally on Debian
• Are you using the Chinese community mirror or a corporate firewall? No

Problem

Our tests are failing when run against the current dev version of the Dart SDK because pub run has started emitting output related to precompilation and we're expecting only output from the executable we're trying to run.

This issue is not present in the current stable SDK.

Expected behavior

pub run <executable> should only emit output from the executable itself.

Actual behavior

pub run <executable> first emits Precompiling executable... and other related lines.

[nex3]

This is also breaking our ability to use the protobuf compiler; see https://travis-ci.com/github/sass/dart-sass-embedded/jobs/363118689.

In general, I would expect an executable runner to never add its own output to stdout (or stderr), since those streams may be used by the underlying executable to communicate along a particular protocol.

[sigurdm]

I believe this is a duplicate of #2525
It was fixed in a8e2442b that should be included in 2.10.0-0.0.dev .

[alextekartik]

Even with "only print if there is a console attached" scenario, if any callers relies on stdout it will break.

In travis for example, My all dart setup (that dart file was writing a .rc file and printing the tmp filename to source) was:

before_script:
  - source $(dart pub run xxxx)

I had to add a warmup calls just to workaround this issue

before_script:
  # Dummy initial call
  - dart pub run xxxx --help
  - source $(dart pub run xxxx)

As I change sdk often, just calling stagehand sometimes displays:

$ stagehand --version
stagehand version: 3.3.9

and sometimes

$ stagehand --version
Can't load Kernel binary: Invalid SDK hash.
Precompiling executable... 
Precompiled stagehand:stagehand.
stagehand version: 3.3.9

Why not printing to stderr instead?