diff -r 000000000000 -r 6474c204b198 js/xpconnect/src/XPCDebug.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/xpconnect/src/XPCDebug.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,134 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* vim: set ts=8 sts=4 et sw=4 tw=99: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "xpcprivate.h"
+#include "jsprf.h"
+#include "js/OldDebugAPI.h"
+
+#ifdef XP_WIN
+#include <windows.h>
+#endif
+
+static void DebugDump(const char* fmt, ...)
+{
+  char buffer[2048];
+  va_list ap;
+  va_start(ap, fmt);
+#ifdef XPWIN
+  _vsnprintf(buffer, sizeof(buffer), fmt, ap);
+#else
+  vsnprintf(buffer, sizeof(buffer), fmt, ap);
+#endif
+  buffer[sizeof(buffer)-1] = '\0';
+  va_end(ap);
+#ifdef XP_WIN
+  if (IsDebuggerPresent()) {
+    OutputDebugStringA(buffer);
+  }
+#endif
+  printf("%s", buffer);
+}
+
+bool
+xpc_DumpJSStack(JSContext* cx, bool showArgs, bool showLocals, bool showThisProps)
+{
+    if (char* buf = xpc_PrintJSStack(cx, showArgs, showLocals, showThisProps)) {
+        DebugDump("%s\n", buf);
+        JS_smprintf_free(buf);
+    }
+    return true;
+}
+
+char*
+xpc_PrintJSStack(JSContext* cx, bool showArgs, bool showLocals,
+                 bool showThisProps)
+{
+    JS::AutoSaveExceptionState state(cx);
+
+    char *buf = JS::FormatStackDump(cx, nullptr, showArgs, showLocals, showThisProps);
+    if (!buf)
+        DebugDump("%s", "Failed to format JavaScript stack for dump\n");
+
+    state.restore();
+    return buf;
+}
+
+/***************************************************************************/
+
+static void
+xpcDumpEvalErrorReporter(JSContext *cx, const char *message,
+                         JSErrorReport *report)
+{
+    DebugDump("Error: %s\n", message);
+}
+
+bool
+xpc_DumpEvalInJSStackFrame(JSContext* cx, uint32_t frameno, const char* text)
+{
+    if (!cx || !text) {
+        DebugDump("%s", "invalid params passed to xpc_DumpEvalInJSStackFrame!\n");
+        return false;
+    }
+
+    DebugDump("js[%d]> %s\n", frameno, text);
+
+    uint32_t num = 0;
+
+    JSAbstractFramePtr frame = JSNullFramePtr();
+
+    JSBrokenFrameIterator iter(cx);
+    while (!iter.done()) {
+        if (num == frameno) {
+            frame = iter.abstractFramePtr();
+            break;
+        }
+        ++iter;
+        num++;
+    }
+
+    if (!frame) {
+        DebugDump("%s", "invalid frame number!\n");
+        return false;
+    }
+
+    JS::AutoSaveExceptionState exceptionState(cx);
+    JSErrorReporter older = JS_SetErrorReporter(cx, xpcDumpEvalErrorReporter);
+
+    JS::RootedValue rval(cx);
+    JSString* str;
+    JSAutoByteString bytes;
+    if (frame.evaluateInStackFrame(cx, text, strlen(text), "eval", 1, &rval) &&
+        nullptr != (str = ToString(cx, rval)) &&
+        bytes.encodeLatin1(cx, str)) {
+        DebugDump("%s\n", bytes.ptr());
+    } else {
+        DebugDump("%s", "eval failed!\n");
+    }
+
+    JS_SetErrorReporter(cx, older);
+    exceptionState.restore();
+    return true;
+}
+
+/***************************************************************************/
+
+JSTrapStatus
+xpc_DebuggerKeywordHandler(JSContext *cx, JSScript *script, jsbytecode *pc,
+                           jsval *rval, void *closure)
+{
+    static const char line[] =
+    "------------------------------------------------------------------------\n";
+    DebugDump("%s", line);
+    DebugDump("%s", "Hit JavaScript \"debugger\" keyword. JS call stack...\n");
+    xpc_DumpJSStack(cx, true, true, false);
+    DebugDump("%s", line);
+    return JSTRAP_CONTINUE;
+}
+
+bool xpc_InstallJSDebuggerKeywordHandler(JSRuntime* rt)
+{
+    return JS_SetDebuggerHandler(rt, xpc_DebuggerKeywordHandler, nullptr);
+}