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.
       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();
 }""");
     }