diff -r 000000000000 -r 6474c204b198 toolkit/devtools/tests/unit/head_devtools.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/devtools/tests/unit/head_devtools.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,43 @@
+"use strict";
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/devtools/Loader.jsm");
+Cu.import("resource://gre/modules/devtools/DevToolsUtils.jsm");
+
+// Register a console listener, so console messages don't just disappear
+// into the ether.
+let errorCount = 0;
+let listener = {
+  observe: function (aMessage) {
+    errorCount++;
+    try {
+      // If we've been given an nsIScriptError, then we can print out
+      // something nicely formatted, for tools like Emacs to pick up.
+      var scriptError = aMessage.QueryInterface(Ci.nsIScriptError);
+      dump(aMessage.sourceName + ":" + aMessage.lineNumber + ": " +
+           scriptErrorFlagsToKind(aMessage.flags) + ": " +
+           aMessage.errorMessage + "\n");
+      var string = aMessage.errorMessage;
+    } catch (x) {
+      // Be a little paranoid with message, as the whole goal here is to lose
+      // no information.
+      try {
+        var string = "" + aMessage.message;
+      } catch (x) {
+        var string = "<error converting error message to string>";
+      }
+    }
+
+    // Make sure we exit all nested event loops so that the test can finish.
+    while (DebuggerServer.xpcInspector.eventLoopNestLevel > 0) {
+      DebuggerServer.xpcInspector.exitNestedEventLoop();
+    }
+    do_throw("head_dbg.js got console message: " + string + "\n");
+  }
+};
+
+let consoleService = Cc["@mozilla.org/consoleservice;1"].getService(Ci.nsIConsoleService);
+consoleService.registerListener(listener);