Allow optional initializing formals again

Allow optional initializing formals again

After removing optional initializing formal parameters from the VM
yesterday, the language team decided that they should be allowed
and made usable.

- The name of an optional initializing formal is the name of
  the field it initializes.
- The parameter is only visible at the call site of the constructor,
  to name the parameter.
- The parameter is NOT visible in the constructor's initializer list
  and body.

class A {
  A([this.x = 10]);
  A.bad([this.x = 10]) : y = x;  // Error: parameter x not in scope
  A.ok([this.x = 10]) { print(x); } // Refers to field x, not parameter x.
  var x, y; 
}

o = new A(x: 50);  // o.x == 50.
o = new A();       // o.x == 10.
Committed: https://code.google.com/p/dart/source/detail?r=1819

[regis, 2011-11-23 22:25:44]

LGTM