[Null-aware elements] Implement flow-analysis for null-aware map entires

[chloestefantsova]

Since the control flow can be changed by the null-aware map entries, the shared flow analysis between the Analyzer and the CFE should be updated accordingly.

Consider the following code:

test1(String a, num x) {
  <String, dynamic>{?a: (x as int)};
  // `x` is promoted to `int`.
}

test2(String? a, num x) {
  <String, dynamic>{?a: (x as int)};
  // `x` is not promoted.
}

In test1 the flow analysis can establish that a is never a null, and the value of the null-aware map entry will always be evaluated; therefore, the promotion of x to int induced by the type cast should be visible below the line with the map literal. In contrast with that, in test2, the flow analysis can't establish that a will never be null, which makes it possible that in some cases x as int will not be executed at all; therefore, the variable x shouldn't be promoted below the line with the map literal.