Adds typechecking of return values

frog/corejs.dart

Index: frog/corejs.dart
diff --git a/frog/corejs.dart b/frog/corejs.dart
index bfa0875c0f5ce9743452b5d5b8df3510cad661b7..4b58b2e3d0c00d9f481f4894655223a7f861fd6c 100644
--- a/frog/corejs.dart
+++ b/frog/corejs.dart
@@ -287,12 +287,10 @@ function $toDartException(e) {
 
     if (useNotNullBool) {
       useThrow = true;
+      // Some testing showed that this patterned fared well across browsers.

[jimhug, 2011/11/11 15:02:02]

Thanks for the comment - I fear that we will have more of these heursitics in
the future and its good to at least have some information on which ones are wild
guesses and which ones had some level of testing behind them.

       w.writeln(@"""
 function $notnull_bool(test) {
-  if (test == null || typeof(test) != 'boolean') {
-    $throw(new TypeError('must be "true" or "false"'));
-  }
-  return test === true;
+  return typeof(test) == 'boolean' ? test : test.is$bool();
 }""");
     }