Fix lower bound of stack_trace

[greglittlefield-wf]

Problem

I hit an error earlier when running tests in a repo whose dependencies hadn't been updated in a while:

NoSuchMethodError: No static method 'capture' with matching arguments declared in class 'Chain'.
Receiver: Type: class 'Chain'
Tried calling: capture(Closure: () => dynamic, when: true)
Found: capture(() => dynamic, {ChainHandler onError})
#0      NoSuchMethodError._throwNew (dart:core-patch/errors_patch.dart:196)
#1      Invoker._onRun (package:test/src/backend/invoker.dart:336)
...

The when parameter to Chain.capture is currently used in invoker.dart#L388, which wasn't added until stack_trace 1.6.0.

Solution

Bump minimum version of stack_trace to ensure this error doesn't happen.

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[greglittlefield-wf]

My company (Workiva) signed it!

[natebosch]

Thanks!