Don't stringify classes without ToStringHelper

[abitofevrything]

Description

Stringifying some classes without ToStringHelper (e.g Stream) results in async errors (e.g Stream.first) that are not caught in stringifyInstance, as well as filling up the console output.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my changes haven't lowered code coverage