[wildcard-variables] Allow super._. (#3845)

* [wildcard-variables] Allow super._.

* Fix comments.

Author: kallentu

working/wildcards/feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom
 
 Status: In-progress
 
-Version 1.4
+Version 1.5
 
 ## Motivation
 
@@ -317,37 +317,45 @@ class C {
 
 ### Super parameters
 
-An occurrence of `super._` as a declaration of a formal parameter in a
-constructor is a compile-time error. This error also occurs in the case
-where the super parameter has an explicitly declared type and/or default
-value.
-
-*`super._` is not an error everywhere: In a method body it could be an
-invocation of an inherited getter named `_`.*
+The positional formal parameter `super._` is still allowed in non-redirecting
+generative constructors. Such a parameter forwards the argument's value to the
+super constructor invocation.
 
 ```dart
-class B {
-  final _;
-  B(this._);
+class A { 
+  final int x, y;
+  A(this.x, this.y);
 }
 
-class C extends B {
-  C(super._); // Error.
+class B extends A {
+  final int _;
+  B(this._, super._, super._) { // No collision on multiple `super._`.
+    print(_ + x + y); // Refers to `B._`, `A.x` and `A.y`. 
+  }
 }
 ```
 
-*The desugared meaning of a super parameter includes a reference to the
-parameter in the initializer list of the enclosing constructor declaration,
-but such references are not possible when the parameter name is a
-wildcard.*
+Similarly to `this._`, and unlike for example `super.x`, a `super._` does not
+introduce any identifier into the scope of the initializer list. 
+
+Because of that, multiple `super._` and `this._` parameters can occur in the
+same constructor without any name collision.
+
+```dart
+class A {
+  final int _, v;
+  A(this._, this.v);
+}
 
-*It may seem inconvenient that `super._` "does not work" in the last
-example, but we do not wish to create exceptions about the ability to refer
-to a declaration whose name is a wildcard, just so we can support this
-particular usage. The advice would be to use a different name than `_` for
-the instance variable in `B` in the first place, or using a different name
-in `B` and possibly `// ignore` a lint that may warn about the names being
-different.*
+class B extends A {
+  B(super.x, super._)
+    : assert(x > 0), // OK, `x` in scope.
+      assert(_ >= 0) // Error: no `_` in scope.
+  { 
+    print(_); // OK, means `this._` and refers to `A._`.
+  }
+}
+```
 
 ### Extension types
 
@@ -461,6 +469,9 @@ This lint is included in the core lint set which means that the scale of the bre
 
 ## Changelog
 
+### 1.5
+- Allow `super._`. Discussion: [language/#3792](https://github.com/dart-lang/language/issues/3792)
+
 ### 1.4
 - Add section on import prefixes. Discussion: [language/#3799](https://github.com/dart-lang/language/issues/3799)