dart2js: more compact encoding of fields that are always initialized to null

[rakudrama]

Many fields are always initialized to null.
We can tell this by looking at the field declarations (are they initialized, and to what) and the generative constructor initializer lists, e.g:

class _StreamImplEvents extends _PendingEvents {
  _DelayedEvent firstPendingEvent = null;
  _DelayedEvent lastPendingEvent = null;
  ...
}

This leads to allocating constructor calls like the following

        t1 = new P._StreamImplEvents(null, null, 0); // all the field initial values.

The synthesized allocating JavaScript constructor looks like this:

  P._StreamImplEvents =
  function _StreamImplEvents(param_firstPendingEvent, param_lastPendingEvent, param_state) {
    this.firstPendingEvent = param_firstPendingEvent;
    this.lastPendingEvent = param_lastPendingEvent;
    this.state = param_state;
  }

It is generated from the field info

  _StreamImplEvents: {
    "^": "_PendingEvents;firstPendingEvent,lastPendingEvent,_state",

The field info could encode the always-null fields, e.g. preceding the field with "+"

  _StreamImplEvents: {
    "^": "_PendingEvents;+firstPendingEvent,+lastPendingEvent,_state",

Now the synthesized constructor has null values in-line:

  P._StreamImplEvents =
  function _StreamImplEvents(param_state) {
    this.firstPendingEvent = null;
    this.lastPendingEvent = null;
    this.state = param_state;
  }

and the calls to the constructor function that omit the always-null parameters:

       t1 = new P._StreamImplEvents(0);

This is a class-local optimization since it is a function of the field declaration initializers and generative constructor initializer lists.

A global version of the optimization would augment these fields with fields with initializers inferred to be null.

[floitschG]

cc @kmillikin.
Removed Type-Defect label.
Added Type-Enhancement, Optimization labels.

[rakudrama]

We no longer 'encode' fields, and the optimization of initializing fields in the constructor is done for simple constants (not just null).