Support for stack traces in catch blocks.

frog/leg/lib/js_helper.dart

Index: frog/leg/lib/js_helper.dart
===================================================================
--- frog/leg/lib/js_helper.dart	(revision 5017)
+++ frog/leg/lib/js_helper.dart	(working copy)
@@ -1303,3 +1303,17 @@
   }
   return ex;
 }
+
+class StackTrace {
+  var stack;
+  StackTrace(this.stack);
+  String toString() => stack != null ? stack : '';
+}
+
+/**
+ * Called by generated code to fetch the stack trace from an
+ * exception.
+ */
+StackTrace getTraceFromException(exception) {
+  return new StackTrace(JS("var", @"$0.stack", exception));
+}