[CFE, Patterns] Type of a constant key is not inferred from rhs of the map pattern

[alexmarkov]

For the following code:

class C<T> {
  const C();
}

void main() {
  var {const C(): a1} = <C<String>, int>{const C<String>(): 1};
}

The type argument of the 1st constant is currently inferred as C<dynamic>, but it should be C<String>.
Kernel:

  static method main() → void {
    dynamic a1;
    {
      final dynamic #0#0 = <bar::C<core::String>, core::int>{#C1: 1};
      if(!(#0#0{core::Map<bar::C<core::String>, core::int>}.{core::Map::length}{core::int} =={core::num::==}{(core::Object) → core::bool} #C2 && #0#0{core::Map<bar::C<core::String>, core::int>}.{core::Map::containsKey}(#C3){(core::Object?) → core::bool} && (let final dynamic #t1 = a1 = #0#0{core::Map<bar::C<core::String>, core::int>}.{core::Map::[]}(#C3){(core::Object?) → dynamic} in true)))
        throw new _in::ReachabilityError::•();
    }
  }


  #C1 = bar::C<core::String> {}
  #C2 = 1
  #C3 = bar::C<dynamic> {}

@munificent kindly provided the following explanation:

We use the matched value's type, Map<C<String>, int> to infer the static type for the map pattern. Then we use the inferred key type from that, C<String> as the context type for the key constant expressions.

co19/LanguageFeatures/Patterns/type_inference_A29_t07 test currently fails with the runtime error because of this bug.

@johnniwinther @chloestefantsova