Implement support for generic function instantiation of existing function objects in constants

[eernstg]

With dart-lang/language#1812, support for generic function instantiation of an existing function object was added to Dart. This issue is concerned with adding support for the same feature for constant expressions. For example:

class A {
  final int Function(int) x;
  const A(bool b): x = (b ? id : other)<int>; // OK, `(...)<T1..Tk>` is potentially constant.
}

X id<X>(X x) => x;
X other<X>(X x) => throw x;

void main() {
  const c1 = id<int>; // Already supported prior to the addition on this feature.
  const c2 = id; // Make `c2` a constant expression whose value is a function object.
  const c3 = c2<int>; // OK, perform generic function instantiation on `c2`.
  const c4 = A(true); // OK, `(b ? id : other)<int>` is constant after substitution `b` -> `true`.
}

Along with these explicit forms, the implicit forms (guided by the context type) should be supported as well.

This feature is part of the constructor-tearoffs feature bundle.

Subtasks:

• CFE issue.
• Analyzer issue.
• DDC issue.
• dart2js issue.
• Write language tests.
• Specification update.

[devoncarew]

@eernstg - just confirming that this is related to constructor tearoffs?

[eernstg]

Yes, the ability to create a generic function instantiation explicitly (e.g., myFunction<int>) was introduced as part of the constructor-tearoffs feature bundle, and this issue is concerned with a part of that feature.

[devoncarew]

Gotch, thanks.