Specify primary constructors (#3023)

This is a proposed feature specification that specifies the feature _primary constructors_. A primary constructor is a syntactically concise notation for a constructor. For instance:

```dart
// Using current Dart.

class Point {
  final int x;
  final int y;
  const Point(this.x, [this.y = 0]);
}

// Using a primary constructor.

class const Point(int x, [int y = 0]);
```

Author: eernstg

LICENSE

@@ -2,7 +2,7 @@ The following license applies to all parts of this repository.
 See https://github.com/dart-lang/sdk/blob/main/LICENSE for the license
 of the Dart language implementation.
 
-Copyright 2018, the Dart project authors. 
+Copyright 2018, the Dart project authors.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
@@ -16,7 +16,7 @@ met:
     * Neither the name of Google LLC nor the names of its
       contributors may be used to endorse or promote products derived
       from this software without specific prior written permission.
-      
+
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR