Treeshaking of sequence expressions

[corneliusweig]

Rollup does not know about the SequenceExpression AST node. This means that the following code is not tree-shaken at all:

// should remove expressions without side-effect
var a = (0,foo(),1,2);
var b = (0,foo());

// should remove variable without effect
var c = (1,2);

// should only keep final expression
var d = (1,2);
console.log(d);

// should infer value
if ((1, 2) !== 2) {
	console.log( 'effect' );
}

Even though it could be simplified to

// IMHO the final value should be retained in this case even though the variable is not used.
var a = (foo(), 2);
var b = foo();

var d = 2;
console.log(d);