Return newValue from indexSet helper and other intercepted setters. Also, throw IllegalArgumentExce…

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 121ef8f5d83a00a194b849c0689214c18a235da4..f22267104fb86120ab410f17beae1d7a5fbd79ef 100644
--- a/dart/frog/leg/lib/js_helper.dart
+++ b/dart/frog/leg/lib/js_helper.dart
@@ -245,7 +245,8 @@ indexSet(var a, var index, var value) {
     return JS("Object", @"$0[$1] = $2", a, index, value);
   }
   checkNull(a);
-  return UNINTERCEPTED(a[index] = value);
+  UNINTERCEPTED(a[index] = value);
+  return value;
 }
 
 builtin$add$1(var receiver, var value) {
@@ -294,6 +295,7 @@ builtin$set$length(receiver, newLength) {
   } else {
     UNINTERCEPTED(receiver.length = newLength);
   }
+  return newLength;
 }
 
 builtin$toString$0(var value) {
@@ -591,6 +593,7 @@ builtin$getRange$2(receiver, start, length) {
   if (start is !int) throw new IllegalArgumentException(start);
   if (length is !int) throw new IllegalArgumentException(length);
   if (start < 0) throw new IndexOutOfRangeException(start);
+  if (length < 0) throw new IllegalArgumentException(length);
   var end = start + length;
   if (end > receiver.length) {
     throw new IndexOutOfRangeException(length);