Document strange downcast conventions used in the library, add a function for downcasts

[osa1]

In protobuf library, for downcasting we use implicit cast syntax:

<type> <var> = <expr>

instead of explicit cast syntax:

[var|final] <var> = <expr> as <type>

The reason is because dart2js compiles these two differently, and the implicit cast syntax is much more efficient.

(AOT and JIT treat both the same way)

For a reader it won't be obvious why we do this. We should create a function for downcasting that uses implicit cast syntax, and document why we use it. @mkustermann suggests

/// Cast value down using implicit `as` check.
/// 
/// This will impact dart2js which does not perform implicit `as` in -O4 (but does perform explicit `as` checks)
@pragma('vm:prefer-inline')
@pragma('dart2js:Inline')
T downcastAs<T>(dynamic value) => value;

[mkustermann]

It may need to be @pragma('dart2js:tryInline'). It could also be written with explicit as check and using @pragma('dart2js:as:trust').

[rakudrama]

Another consideration is the debug performance in DDC which never inlines methods.
It might be better to put @pragma('dart2js:as:trust') on the method that contains the cast rather than add an 'abstraction'.

I recently added some documentation of the dart2js pragmas:
https://dart.googlesource.com/sdk/+/f503281917b74ac0571bd4bc5c37789006cdef8e/pkg/compiler/doc/pragmas.md

[osa1]

In #1019 we experimented with encapsulating this pattern in a function like downcastAs shown in the original issue. Unfortunately inlining is done too late in AOT and dart2wasm (later than TFA), which cause performance issues. In the short term we have no option other than just learning about these two ways of type casting and suppressing the lints.