Add modifier for static variables and methods to be inherited by all subclasses.

Static variables do not pass thru in the inheritance like instance variables. Not sure why Dart architecture does it like that. I know for example in Smalltalk ClassVariableNames are passed thru (via methods) automatically. However I believe, in Dart, this is a limitation in that to create a subclass, one must also implement access to static variables:

E.g.
```dart
// Parent widget with static variable and method
class PWidget {
static int height = 5;

static _width = 10;
static int get width => _width;
static set width(int value) {
_width=value;
}

}

// Child Widget. However static of parent needs to be reimplemented to be accessible.
class CWidget extends PWidget {
static int height = PWidget.height;
}

main() {
print(PWidget.height);
print(CWidget.height); // works because it is implemented in in child class
}
```
This imposes a responsibility on someone creating a subclass to reimplement all the static variables so that they are accessible by the user of the subclass. A simpler way of course would be to pass thru the static variable and methods to the child class, without the need to reimplement.

I propose to give the designer the ability to subclass all static parent class variables AND methods using a static modifier on the class creation. E.g.

```dart
// Parent widget
class PWidget {
static int height = 5;

static _width = 10;
static int get width => _width;
static set width(int value) {
_width=value;
}
}

// Child Widget extends PWidget with a static modifer
class CWidget extends PWidget static {

}

main() {
print(PWidget.height);
print(CWidget.height);
print(CWidget.width);
CWidget.width = 50;
}
```