[fix] #631810: Form.DialogResult needs to call its close events *before* closing.

	* Form.cs: When setting DialogResult call RaiseCloseEvents
	instead of directly setting 'closing', since we need to have our
	FormClosing event invoked *before* the form dissapears, so the
	user can properly cancel the close operation if desired.
	Fixes #631810.

mcs/class/Managed.Windows.Forms/System.Windows.Forms/Form.cs

@@ -708,7 +708,8 @@ namespace System.Windows.Forms {
 							typeof (DialogResult));
 
 				dialog_result = value;
-				closing = (dialog_result != DialogResult.None && is_modal);
+				if (dialog_result != DialogResult.None && is_modal)
+					RaiseCloseEvents (false, false); // .Net doesn't send WM_CLOSE here.
 			}
 		}