Ensure that all opened editors' buffers are added to the project

Assigning a language mode happens when adding a buffer to the project,
so we need to guarantee this happens to all buffers used by visible editors.

spec/workspace-spec.js

@@ -1,6 +1,7 @@
 const path = require('path')
 const temp = require('temp').track()
 const dedent = require('dedent')
+const TextBuffer = require('text-buffer')
 const TextEditor = require('../src/text-editor')
 const Workspace = require('../src/workspace')
 const Project = require('../src/project')
@@ -933,6 +934,18 @@ describe('Workspace', () => {
         })
       })
     })
+
+    describe('when opening an editor with a buffer that isn\'t part of the project', () => {
+      it('adds the buffer to the project', async () => {
+        const buffer = new TextBuffer()
+        const editor = new TextEditor({buffer})
+
+        await atom.workspace.open(editor)
+
+        expect(atom.project.getBuffers().map(buffer => buffer.id)).toContain(buffer.id)
+        expect(buffer.getLanguageMode().getLanguageId()).toBe('text.plain.null-grammar')
+      })
+    })
   })
 
   describe('finding items in the workspace', () => {

src/workspace.js

@@ -497,6 +497,9 @@ module.exports = class Workspace extends Model {
           this.textEditorRegistry.maintainConfig(item),
           item.observeGrammar(this.handleGrammarUsed.bind(this))
         )
+        if (!this.project.findBufferForId(item.buffer.id)) {
+          this.project.addBuffer(item.buffer)
+        }
         item.onDidDestroy(() => { subscriptions.dispose() })
         this.emitter.emit('did-add-text-editor', {textEditor: item, pane, index})
       }