chore(lint): Enable no-useless-concat.

.eslintrc

@@ -136,6 +136,7 @@
     "no-unneeded-ternary": 2,
     "no-unused-vars": [2, {"vars": "all", "args": "none"}],
     "no-useless-call": 2,
+    "no-useless-concat": 2,
     "no-useless-constructor": 2,
     "no-var": 2,
     "no-void": 2,

content-src/components/Search/Search.js

@@ -45,7 +45,7 @@ const Search = React.createClass({
     const numSuggestions = this.props.formHistory.concat(this.props.suggestions).length;
     const numEngines = this.props.engines.length;
     if (index < numSuggestions) {
-      return "search-suggestions-" + "history-search-suggestions-" + index;
+      return `${index < this.props.formHistory.length ? "history-" : ""}search-suggestions-${index}`;
     } else if (index < numSuggestions + numEngines) {
       return "search-partners-" + (index - numSuggestions);
     } else if (index === numSuggestions + numEngines) {