Enable 'primary-constructors' feature flag.

This CL enables the primary constructors feature by default in Dart 3.13.

The primary constructors feature is a brevity feature. There are no new semantics, but it allows us to express declarations in a less verbose way.

This feature allows one constructor and a set of instance variables to be specified in the header of a declaration.

Currently a declaration with a constructor and some fields is written as:

```dart
// Current syntax.
class Point {
  int x;
  int y;
  Point(this.x, this.y);
}
```

With a primary constructor, we would write the above as:

```
class Point(var int x, var int y);
```

If a primary constructor needs an initializer list or a body, they can be
specified inside the class using the `this` body syntax:

```dart
class Point(var int x, var int y) {
  this : assert(x >= 0) {
    print('Point created at $x, $y');
  }
}
```

As part of this feature, you can also use the `new` and `factory` keywords to
declare constructors in the class body without repeating the class name:
```dart
class Point {
  int x, y;

  // Equivalent to Point(this.x, this.y)
  new(this.x, this.y);

  // Equivalent to Point.origin()
  new origin() : x = 0, y = 0;

  // Equivalent to factory Point.clone(Point other)
  factory clone(Point other) => Point(other.x, other.y);
}
```

To learn more about the feature, check out the feature specification located here: https://github.com/dart-lang/language/blob/main/accepted/future-releases/primary-constructors/feature-specification.md

Tested: Has existing language, CFE, analyzer, analysis server tests.
Bug: https://github.com/dart-lang/sdk/issues/61524
Change-Id: I296f2fcd918b87bf2a1dd00256340759866c2423
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/489241
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Michael Thomsen <mit@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>

tools/experimental_features.yaml

@@ -164,10 +164,6 @@ features:
   static-extensions:
     help: "Extensions with static capabilities."
 
-  primary-constructors:
-    help: "Less verbose constructors."
-    experimentalReleaseVersion: '3.12.0'
-
   anonymous-methods:
     help: "Anonymous methods."
     # TODO(eernst): Enable this when the feature is implemented.
@@ -188,6 +184,16 @@ features:
 #
 # Shipped flags should be marked retired the following stable release.
 #
+  primary-constructors:
+    enabledIn: '3.13.0'
+    help: "Less verbose constructors."
+    experimentalReleaseVersion: '3.12.0'
+    validation: |
+      class C(var String x);
+      main() {
+        print(C('feature enabled').x);
+      }
+
   private-named-parameters:
     enabledIn: '3.12.0'
     help: "Allow named parameters with private names that refer to fields."