Prevent sort on immutable lists.

dart/frog/leg/lib/js_helper.dart

Index: dart/frog/leg/lib/js_helper.dart
diff --git a/dart/frog/leg/lib/js_helper.dart b/dart/frog/leg/lib/js_helper.dart
index ca78471a909b3163ed3598c2f1b89b0d453dd6d8..b83a9667c36e1495b73da90f60a9a8cea85b3c8b 100644
--- a/dart/frog/leg/lib/js_helper.dart
+++ b/dart/frog/leg/lib/js_helper.dart
@@ -784,6 +784,7 @@ builtin$every$1(receiver, f) {
 builtin$sort$1(receiver, compare) {
   if (!isJsArray(receiver)) return UNINTERCEPTED(receiver.sort(compare));
 
+  checkMutable(receiver, 'sort');
   DualPivotQuicksort.sort(receiver, compare);
 }
 
@@ -1363,3 +1364,11 @@ class Null {
     throw UnsupportedOperationException('new Null()');
   }
 }
+
+builtin$copyFrom$4(receiver, arg1, arg2, arg3, arg4) {
+  // TODO(ahe): Remove this method when CL 9839101 has landed.
+  if (!isJsArray(receiver)) {
+    return UNINTERCEPTED(receiver.copyFrom(arg1, arg2, arg3, arg4));
+  }
+  throw new UnsupportedOperationException('copyFrom');
+}