Move the depth guard to the internal flatten

Manual testing revealed that flattening with depth=false behaved as a
shallow flatten instead of a deep one.

modules/_flatten.js

@@ -6,6 +6,11 @@ import isArguments from './isArguments.js';
 // Internal implementation of a recursive `flatten` function.
 export default function flatten(input, depth, strict, output) {
   output = output || [];
+  if (!depth && depth !== 0) {
+    depth = Infinity;
+  } else if (depth <= 0) {
+    return output.concat(input);
+  }
   var idx = output.length;
   for (var i = 0, length = getLength(input); i < length; i++) {
     var value = input[i];

modules/flatten.js

@@ -1,13 +1,7 @@
 import _flatten from './_flatten.js';
-import clone from './clone.js';
 
 // Flatten out an array, either recursively (by default), or up to `depth`.
 // Passing `true` or `false` as `depth` means `1` or `Infinity`, respectively.
 export default function flatten(array, depth) {
-  if (!depth && depth !== 0) {
-    depth = Infinity;
-  } else if (depth <= 0) {
-    return clone(array);
-  }
   return _flatten(array, depth, false);
 }