Linter: `avoid_dynamic_calls` should allow `<any>.runtimeType`

[matanlurey]

See flutter/engine#53228 (comment).

This was just an oversight and I want to fix it :)

[bwilkerson]

Given that the lint's documentation reads

There is an exception to methods and properties that exist on Object?:

• a.hashCode
• a.runtimeType
• a.noSuchMethod(someInvocation)
• a.toString()

It does seem like this is a bug.

[srawlins]

Fixed with 28874ee