Copy JSSyntaxRegExp from frog.

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 6d85849d5770801e6a27b2473bbc5d0c2bd4789a..59919d74c956ac7924037936e67dc0132f509c93 100644
--- a/dart/frog/leg/lib/js_helper.dart
+++ b/dart/frog/leg/lib/js_helper.dart
@@ -7,6 +7,7 @@
 #import('coreimpl.dart');
 
 #source('date_helper.dart');
+#source('regexp_helper.dart');
 
 /**
   * Returns true if both arguments are numbers.
@@ -799,6 +800,12 @@ checkBool(value) {
   return value;
 }
 
+checkString(value) {
+  checkNull(value);

[kasperl, 2012/02/21 07:35:17]

Would it be simpler to rewrite some of these to do a positive is test instead?
Something like:

   if (value is String) return value;
   throw new IllegalArgumentException(value);

That way you don't need the explicit null check.

+  if (value is !String) throw new IllegalArgumentException(value);
+  return value;
+}
+
 builtin$isNegative$0(receiver) {
   checkNull(receiver);
   if (receiver is num) {