In the US at least, the name Dashiell is so closely associated with the writer Dashiell Hammett, that pronouncing it any other way would sound silly, especially since Dashiell is considered a bit of a yuppy hipster name, going against the way Dashiell Hammett's name was pronounced takes away that much of that cool and hip appeal. .
According to a Dashiell Hammett site.
How the heck do you pronounce Hammett’s first name?
“Dashiell” was the Americanized spelling of an old family name and was pronounced “deh-SHYEEL.” Radio announcers further Americanized it to “DASH-ull.” Most of Hammett’s friends by this time simply called him “Dash.”
http://www.mikehumbert.com/Dashiell_Hammett_01_Short_Bio.html
Plus in the US you have other similar last names such as Daschle, as in former Sen Tom Daschle's last name, that are also pronounced Dash-ull. Probably not the correct pronunciation, but in the US that's how we say the name.