Kernel text printer doesn't properly parenthesize ((...) => expr) as type

[stereotype441]

This arose during a code review: https://dart-review.googlesource.com/c/sdk/+/20907/1/pkg/front_end/testcases/inference/generic_methods_inference_error.dart.strong.expect#7

When an AsExpression contains a FunctionExpression, the text printer prints it as (...) => expr as type, which makes it look like the FunctionExpression contains the AsExpression. The text printer should add parentheses: ((...) => expr) as type.

It looks like it should be a pretty simple fix--I'll work on it if time allows. If someone on the kernel team wants to take over this bug from me, please feel free.