Bug 1394849 - Add error.stack to create stacktraces. r=automatedtester

This patch introduces a new error.stack function as a shorthand for
creating stacktraces.  It is equivalent to calling

	new Error().stack

and removing the first line of the stack.  Removing the first line is
needed to make it appear as if the error originated from the caller's
position in the program.

MozReview-Commit-ID: DpSSWU5vPDm

--HG--
extra : rebase_source : 52697e348367b2b7dbeb28a711a9bd1fdef076ec

testing/marionette/error.js

@@ -48,6 +48,7 @@ const BUILTIN_ERRORS = new Set([
 this.EXPORTED_SYMBOLS = [
   "error",
   "pprint",
+  "stack",
 ].concat(Array.from(ERRORS));
 
 /** @namespace */
@@ -215,6 +216,14 @@ this.pprint = function(ss, ...values) {
   return res.join("");
 };
 
+/** Create a stacktrace to the current line in the program. */
+this.stack = function() {
+  let trace = new Error().stack;
+  let sa = trace.split("\n");
+  sa = sa.slice(1);
+  return "stacktrace:\n" + sa.join("\n");
+};
+
 /**
  * WebDriverError is the prototypal parent of all WebDriver errors.
  * It should not be used directly, as it does not correspond to a real

testing/marionette/test_error.js

@@ -24,6 +24,7 @@ const {
   pprint,
   ScriptTimeoutError,
   SessionNotCreatedError,
+  stack,
   StaleElementReferenceError,
   TimeoutError,
   UnableToSetCookieError,
@@ -144,6 +145,14 @@ add_test(function test_pprint() {
   run_next_test();
 });
 
+add_test(function test_stack() {
+  equal("string", typeof stack());
+  ok(stack().includes("test_stack"));
+  ok(!stack().includes("add_test"));
+
+  run_next_test();
+});
+
 add_test(function test_toJSON() {
   let e0 = new WebDriverError();
   let e0s = e0.toJSON();