Adds typechecking of return values

frog/lib/corelib_impl.dart

Index: frog/lib/corelib_impl.dart
diff --git a/frog/lib/corelib_impl.dart b/frog/lib/corelib_impl.dart
index 3b249203ac22a781baa6a88a2f1e21d264f37bc8..e08e38c59950532fc4da9255666a21dee20e270f 100644
--- a/frog/lib/corelib_impl.dart
+++ b/frog/lib/corelib_impl.dart
@@ -235,9 +235,9 @@ class JSSyntaxRegExp implements RegExp {
   final bool multiLine;
   final bool ignoreCase;
 
-  const JSSyntaxRegExp(String pattern,
-      [bool multiLine = false, bool ignoreCase = false]):
-    this._create(pattern, (multiLine ? 'm' : '') + (ignoreCase ? 'i' : ''));
+  const JSSyntaxRegExp(String pattern, [bool multiLine, bool ignoreCase]):

[Jennifer Messerly, 2011/11/11 02:36:01]

this is an unrelated fix I noticed in co19 checked mode (which isn't running
yet, but anyway...)

"null" is allowed as the default arg & shouldn't cause a type error.

+    this._create(pattern,
+        (multiLine == true ? 'm' : '') + (ignoreCase == true ? 'i' : ''));
 
   const JSSyntaxRegExp._create(String pattern, String flags) native
     '''this.re = new RegExp(pattern, flags);