Override global jasmine spec functions

Currently, if a spec uses the global `it` function on an async test,
that test will always pass (since the jasmine version checked in Atom
does not natively support tests that return promises). This can be
confusing since the test behaviour is different between the
async-test-helpers methods and the global ones.

By overriding the global functions, we'll also be able to remove all the
imports from async-test-helpers since they won't be needed anymore.

More info: https://github.com/atom/atom/pull/18896#discussion_r260396102

spec/jasmine-test-runner.coffee

@@ -10,6 +10,16 @@ module.exports = ({logFile, headless, testPaths, buildAtomEnvironment}) ->
   window[key] = value for key, value of require '../vendor/jasmine'
   require 'jasmine-tagged'
 
+  # Rewrite global jasmine functions to have support for async tests.
+  # This way packages can create async specs without having to import these from the
+  # async-spec-helpers file.
+  global.it = asyncifyJasmineFn global.it, 1
+  global.fit = asyncifyJasmineFn global.fit, 1
+  global.ffit = asyncifyJasmineFn global.ffit, 1
+  global.fffit = asyncifyJasmineFn global.fffit, 1
+  global.beforeEach = asyncifyJasmineFn global.beforeEach, 0
+  global.afterEach = asyncifyJasmineFn global.afterEach, 0
+
   # Allow document.title to be assigned in specs without screwing up spec window title
   documentTitle = null
   Object.defineProperty document, 'title',
@@ -59,6 +69,28 @@ module.exports = ({logFile, headless, testPaths, buildAtomEnvironment}) ->
   jasmineEnv.execute()
   promise
 
+asyncifyJasmineFn = (fn, callbackPosition) ->
+  (args...) ->
+    if typeof args[callbackPosition] is 'function'
+      callback = args[callbackPosition]
+
+      args[callbackPosition] = (args...) ->
+        result = callback.apply this, args
+        if result instanceof Promise
+          waitsForPromise(-> result)
+
+    fn.apply this, args
+
+waitsForPromise = (fn) ->
+  promise = fn()
+
+  global.waitsFor('spec promise to resolve', (done) ->
+    promise.then(done, (error) ->
+      jasmine.getEnv().currentSpec.fail error
+      done()
+    )
+  )
+
 disableFocusMethods = ->
   ['fdescribe', 'ffdescribe', 'fffdescribe', 'fit', 'ffit', 'fffit'].forEach (methodName) ->
     focusMethod = window[methodName]
@@ -124,4 +156,4 @@ buildTerminalReporter = (logFile, resolveWithExitCode) ->
     new JasmineListReporter(options)
   else
     {TerminalReporter} = require 'jasmine-tagged'
-    new TerminalReporter(options)
+    new TerminalReporter(options)