override chrome:// URLs via extensions.

chrome/test/data/extensions/api_test/override1/api_test.js

+// api_test.js
+// mini-framework for ExtensionApiTest browser tests
+// TODO(erikkay) - figure out a way to share this code across extensions
+
+var completed = false;
+var tests;
+var currentTest;
+
+function complete() {
+  completed = true;
+
+  // a bit of a hack just to try to get the script to stop running at this point
+  throw "completed";
+}
+
+function fail(message) {
+  if (completed) throw "completed";
+
+  var stack;
+  try {
+    crash.me += 0;  // intentional exception to get the stack trace
+  } catch (e) {
+    stack = e.stack.split("\n");
+    stack = stack.slice(2);  // remove title and fail()
+    stack = stack.join("\n");
+  }
+
+  if (!message) {
+    message = "FAIL (no message)";
+  }
+  message += "\n" + stack;
+  console.log("[FAIL] " + currentTest.name + ": " + message);
+  chrome.test.fail(message);
+  complete();
+}
+
+function allTestsSucceeded() {
+  console.log("All tests succeeded");
+  if (completed) throw "completed";
+
+  chrome.test.pass();
+  complete();
+}
+
+function runNextTest() {
+  currentTest = tests.shift();
+  if (!currentTest) {
+    allTestsSucceeded();
+    return;
+  }
+  currentTest.call();
+}
+
+function succeed() {
+  console.log("[SUCCESS] " + currentTest.name);
+  runNextTest();
+}
+
+window.onerror = function(message, url, code) {
+  if (completed) return;
+
+  fail(message);
+};
+
+function assertTrue(test, message) {
+  if (test !== true) {
+    if (typeof(test) == "string") {
+      if (message) {
+        message = test + "\n" + message;
+      } else {
+        message = test;
+      }
+    }
+    fail(message);
+  }
+}
+
+function assertNoLastError() {
+  if (chrome.extension.lastError != undefined) {
+    fail("lastError.message == " + chrome.extension.lastError.message);
+  }
+}
+
+// Wrapper for generating test functions, that takes care of calling
+// assertNoLastError() and succeed() for you.
+function testFunction(func) {
+  return function() {
+    assertNoLastError();
+    try {
+      func.apply(null, arguments);
+    } catch (e) {
+      var stack = null;
+      if (typeof(e.stack) != "undefined") {
+        stack = e.stack.toString()
+      }
+      var msg = "Exception during execution of testFunction in " +
+                currentTest.name;
+      if (stack) {
+        msg += "\n" + stack;
+      } else {
+        msg += "\n(no stack available)";
+      }
+      fail(msg);
+    }
+    succeed();
+  };
+}