[dart2js] Basic chaining of field assignments

      t4 = t3.tooltip;
      t2._tooltipText = t4;
      this.currenttooltip = t4;
--->
      this.currenttooltip = t2._tooltipText = t3.tooltip;


        future = new P._Future(0, $.Zone__current, [P.bool]);
        this._stateData = future;
        return future;
--->
        return this._stateData = new P._Future(0, $.Zone__current, [P.bool]);

Change-Id: I8a1c4dae85b8a10f8b2c099668033f0c1ea9b6c4
Reviewed-on: https://dart-review.googlesource.com/c/93342
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>