Add functionality to get full stack trace when exceptions are thrown.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
===================================================================
--- runtime/lib/errors_patch.dart	(revision 19025)
+++ runtime/lib/errors_patch.dart	(working copy)
@@ -150,3 +150,12 @@
     return msg_buf.toString();
   }
 }
+
+patch class Stacktrace {
+  /* patch */ String get fullStacktrace native "Stacktrace_getFullStacktrace";
+
+  /* patch */ String get stacktrace native "Stacktrace_getStacktrace";
+
+  /* patch */ void _setupFullStacktrace()
+      native "Stacktrace_setupFullStacktrace";
+}