Make OverReact strong-mode compliant

[jacehensley-wf]

• Issue Type: FEATURE REQUEST

We should make OverReact strong-mode compliant. With all the new DDC stuff coming out including the Dart Console, it would be really nice to let our consumers use those new tools.

• Written code is strong-mode compliant - UIP-1637 First pass at strong mode compliance #15
• Transformed code is strong-mode compliant

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FYI: @greglittlefield-wf @aaronlademann-wf @clairesarsam-wf @joelleibow-wf

[jacehensley-wf]

Related issue, WRT transformed code, of what seems like a strong-mode bug: dart-lang/sdk#28369

Scratch that, that warning is valid.