Tree: Item "expand" button must have discernible text within the icon

http://design-system-react-components.herokuapp.com/?selectedKind=SLDSTree&selectedStory=Base&full=0&addons=1&stories=1&panelRight=0&addonPanel=storybook%2Factions%2Factions-panel
(error occurs in all Trees states except Overflow Hidden and Large dataset)

Correction is necessary because assistive text is missing (even though item 1 of 4 does not appear as expandable)

HTML: <div class="slds-tree__item slds-is-selected"><button class="slds-button slds-button_icon slds-m-right_small slds-is-disabled" disabled="" tabindex="-1" type="button" aria-hidden="true"><svg aria-hidden="true" class="slds-button__icon slds-button__icon_small"><use xlink:href="/assets/icons/utility-sprite/svg/symbols.svg#chevronright"></use></svg></button><span class="slds-size_1-of-1"><span class="slds-tree__item-label slds-truncate" title="Grains">Grains</span></span></div>

Add: <span class="slds-assistive-text">Expand Grains</span> between </use> and </button>

Corrected html: <div class="slds-tree__item slds-is-selected"><button class="slds-button slds-button_icon slds-m-right_small slds-is-disabled" disabled="" tabindex="-1" type="button" aria-hidden="true"><svg aria-hidden="true" class="slds-button__icon slds-button__icon_small"><use xlink:href="/assets/icons/utility-sprite/svg/symbols.svg#chevronright"></use></svg><span class="slds-assistive-text">Expand Grains</span></button><span class="slds-size_1-of-1"><span class="slds-tree__item-label slds-truncate" title="Grains">Grains</span></span></div>

Success Criteria: component states will pass automated aXe test.