Fix Runtime_ArrayConcat to handle FAST_DOUBLE_ELEMENTS

test/mjsunit/elements-kind.js

Index: test/mjsunit/elements-kind.js
diff --git a/test/mjsunit/elements-kind.js b/test/mjsunit/elements-kind.js
index cfd47c7781203078ecf7a3892cfa9abbe8ff62fa..4790a618412f5755dddc3a5c4f584c50aef92101 100644
--- a/test/mjsunit/elements-kind.js
+++ b/test/mjsunit/elements-kind.js
@@ -305,5 +305,17 @@ if (support_smi_only_arrays) {
   assertTrue(%HaveSameMap(e, f));
 }
 
+// Test if Array.concat() works correctly with DOUBLE elements.
+if (support_smi_only_arrays) {
+  var a = [1, 2];
+  assertKind(elements_kind.fast_smi_only, a);
+  var b = [4.5, 5.5];
+  assertKind(elements_kind.fast_double, b);
+  var c = a.concat(b);
+  assertEquals([1, 2, 4.5, 5.5], c);
+  // TODO(1810): Change implementation so that we get DOUBLE elements here?
+  assertKind(elements_kind.fast, c);
+}
+
 // Throw away type information in the ICs for next stress run.
 gc();