Use const constructor for ParagraphConstraints

[cbracken]

Usages of ParagraphConstraints (from dart:ui) whose constructor could be const as of flutter/engine#7346 are currently marked // ignore:prefer_const_constructors in the framework until all Google-internal embedders have been updated to an engine version that includes the above change. These were initially updated in engine roll #26252, but broke internal embedders so were reverted in #26392.

We should re-enable use of the const constructor in those cases once internal embedders are updated.

[github-actions]

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