Important: as Blender 2.8 is still in development the following may be subject to change.
With Blender 2.8 comes a significant change to selection behaviour. The previous options, Inverse and (De)select All, have been updated so the Select menu now includes three distinct options; All, None and Invert. This change applies to all Editors, Workspaces and Areas as it ostensibly replaces (De)select All.
Design note: in essence (De)select All (see below) has been split into two distinct actions, one to select (Select All) and another to deselect (Deselect All), minimizing the issue of inadvertent menu or shortcut overuse (pressing the selection key several times, a consequence of using a single action to perform multiple functions whilst not knowing the exact state a mesh or scene might be in before attempting to de/select items).
Separate De/Select All
The default selection behaviour comprises three shortcut combinations, one key or combination per action, these are; All (shortcut A) selects everything regardless of any current selection/s; None (shortcut Alt+A) deselects everything regardless of current selection/s; Invert (shortcut Ctrl+I) inverts current selection/s, i.e. if one side of a cube is selected, Invert deselects the initial face and selects everything not previously selected.
A - selects everything (Select All) Alt+A - deselects everything (Deselect All) Ctrl+I - inverts selection (deselects active/selects inactive)
If a selection has been made  (in any Interactive Context mode i.e. Object Mode, Edit Mode etc.) but ‘all’ is required , choosing the option  (by menu or shortcut key) will do this explicitly regardless – active selections no longer need to be cleared beforehand.
The dual select/deselect function of (De)select All have been separated, each is now a distinct operation that will do one or the other explicitly.
Single key (De)select All
With different keyboard shortcut combinations set to select or deselect objects by default, it is still possible to use the single-key (De)select All selection action of previous versions of Blender. To do this access (User) Preferences from Edit » Preferences . In Blender Preferences click Keymap  then under Preferences activate (click) the Select All Toggles checkbox . Close the Blender Preferences window; (De)select All toggle will now be active.
Design note: to make the change permanent, after enabling the setting click Save Preferences bottom-left to store the property in the Start-up file (make sure to be using the ‘default’ Scene – Cube, Lamp, Camera – when modifying Preferences and saving them otherwise whatever is in the Scene or open project will be loaded every time Blender starts).
To enable the (De)select All action previously attributed to using the A key alone, enable Select All Toggles  in Preferences  – object selection will then be determined on the A key to select or deselect everything.