assign(lvalue, rvalue) {
    let binding = lvalue instanceof BindingElement ?
        lvalue.binding : createBindingIdentifier(lvalue);
    rvalue = this.createInitializer(rvalue);
    this.expressions.push(createVariableDeclaration(binding, rvalue));
  }
@param {BindingElement|IdentifierExpression} lvalue
@param {ParseTree} rvalue