fn.dart shouldn't spam the log
fn.dart shouldn't spam the log

This CL introduces a flag to control whether fn.dart prints the render duration
to the console. By default, the flag is off to avoid log spam.

Also, improve the way we intialize _isInCheckedMode to make use of the fact
that Dart intializes final fields lazily.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/982073002

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