Merge pull request #2898 from vepor/css-without-interpolations

Unnecessary flattening and interleave of css without interpolations

packages/styled-components/src/constructors/css.js

@@ -12,6 +12,11 @@ export default function css(styles: Styles, ...interpolations: Array<Interpolati
     return flatten(interleave(EMPTY_ARRAY, [styles, ...interpolations]));
   }
 
+  if(interpolations.length === 0 && styles.length === 1 && typeof styles[0] === "string") {
+    // $FlowFixMe
+    return styles;
+  }
+
   // $FlowFixMe
   return flatten(interleave(styles, interpolations));
 }