sankey move defaultLinkColor outside for loop to speed up coerce

Author: antoinerg

Diff for: src/traces/sankey/defaults.js

@@ -56,10 +56,12 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
     coerceLink('hoverinfo');
     handleHoverLabelDefaults(linkIn, linkOut, coerceLink, layout.hoverlabel);
 
+    var defautLinkColor = tinycolor(layout.paper_bgcolor).getLuminance() < 0.333 ?
+                'rgba(255, 255, 255, 0.6)' :
+                'rgba(0, 0, 0, 0.2)';
+
     coerceLink('color', linkOut.value.map(function() {
-        return tinycolor(layout.paper_bgcolor).getLuminance() < 0.333 ?
-            'rgba(255, 255, 255, 0.6)' :
-            'rgba(0, 0, 0, 0.2)';
+        return defautLinkColor;
     }));
 
     handleDomainDefaults(traceOut, layout, coerce);