Add atom.whenShellEnvironmentLoaded()

This provides a way to be notified when the shell environment has
loaded—even if it's already happened.

spec/atom-environment-spec.coffee

@@ -458,6 +458,39 @@ describe "AtomEnvironment", ->
       atomEnvironment.unloadEditorWindow()
       atomEnvironment.destroy()
 
+  describe "::whenShellEnvironmentLoaded()", ->
+    [atomEnvironment, envLoaded, spy] = []
+
+    beforeEach ->
+      resolve = null
+      promise = new Promise (r) -> resolve = r
+      envLoaded = ->
+        resolve()
+        waitsForPromise -> promise
+      atomEnvironment = new AtomEnvironment
+        applicationDelegate: atom.applicationDelegate
+        updateProcessEnv: -> promise
+      atomEnvironment.initialize({window, document})
+      spyOn(atomEnvironment.packages, 'getAvailablePackagePaths').andReturn []
+      spyOn(atomEnvironment, 'displayWindow').andReturn Promise.resolve()
+      spy = jasmine.createSpy()
+      atomEnvironment.startEditorWindow()
+
+    afterEach ->
+      atomEnvironment.unloadEditorWindow()
+      atomEnvironment.destroy()
+
+    it "is triggered once the shell environment is loaded", ->
+      atomEnvironment.whenShellEnvironmentLoaded spy
+      envLoaded()
+      runs -> expect(spy).toHaveBeenCalled()
+
+    it "triggers the callback immediately if the shell environment is already loaded", ->
+      envLoaded()
+      runs ->
+        atomEnvironment.whenShellEnvironmentLoaded spy
+        expect(spy).toHaveBeenCalled()
+
   describe "::openLocations(locations) (called via IPC from browser process)", ->
     beforeEach ->
       spyOn(atom.workspace, 'open')

src/atom-environment.coffee

@@ -125,7 +125,7 @@ class AtomEnvironment extends Model
 
   # Call .loadOrCreate instead
   constructor: (params={}) ->
-    {@applicationDelegate, @clipboard, @enablePersistence, onlyLoadBaseStyleSheets} = params
+    {@applicationDelegate, @clipboard, @enablePersistence, onlyLoadBaseStyleSheets, @updateProcessEnv} = params
 
     @nextProxyRequestId = 0
     @unloaded = false
@@ -136,6 +136,7 @@ class AtomEnvironment extends Model
     @deserializeTimings = {}
     @views = new ViewRegistry(this)
     @notifications = new NotificationManager
+    @updateProcessEnv ?= updateProcessEnv # For testing
 
     @stateStore = new StateStore('AtomEnvironments', 1)
 
@@ -398,6 +399,17 @@ class AtomEnvironment extends Model
   onDidFailAssertion: (callback) ->
     @emitter.on 'did-fail-assertion', callback
 
+  # Extended: Invoke the given callback as soon as the shell environment is
+  # loaded (or immediately if it was already loaded).
+  #
+  # * `callback` {Function} to be called whenever there is an unhandled error
+  whenShellEnvironmentLoaded: (callback) ->
+    if @shellEnvironmentLoaded
+      callback()
+      new Disposable()
+    else
+      @emitter.once 'loaded-shell-environment', callback
+
   ###
   Section: Atom Details
   ###
@@ -660,8 +672,10 @@ class AtomEnvironment extends Model
   # Call this method when establishing a real application window.
   startEditorWindow: ->
     @unloaded = false
-    updateProcessEnvPromise = updateProcessEnv(@getLoadSettings().env)
+    updateProcessEnvPromise = @updateProcessEnv(@getLoadSettings().env)
     updateProcessEnvPromise.then =>
+      @shellEnvironmentLoaded = true
+      @emitter.emit('loaded-shell-environment')
       @packages.triggerActivationHook('core:loaded-shell-environment')
 
     loadStatePromise = @loadState().then (state) =>