Remove ember-source from devDependencies.

We were using it primarily to test that our
`FastBootHeaders.prototype.unknownProperty` method functioned
properly. This refactors things to use a simulated `Ember.get` to allow
the same test scenario to succeeed, but avoid having to depend on Ember
itself.

Note: this does not remove any "real" testing of various Ember versions,
most of our tests are using fixtures with a fully compiled ember-cli
applications.

Author: Robert Jackson

package.json

@@ -36,7 +36,6 @@
   "devDependencies": {
     "chai": "^4.2.0",
     "chai-as-promised": "^7.1.1",
-    "ember-source": "3.8.0",
     "eslint": "^6.5.1",
     "eslint-config-prettier": "^6.4.0",
     "eslint-plugin-chai-expect": "^2.0.1",

test/fastboot-headers-test.js

@@ -2,7 +2,6 @@
 
 var expect = require('chai').expect;
 var FastBootHeaders = require('./../src/fastboot-headers.js');
-var Ember = require('ember-source/dist/ember.debug');
 
 describe('FastBootHeaders', function() {
   it('lower normalizes the headers to lowercase', function() {
@@ -161,6 +160,19 @@ describe('FastBootHeaders', function() {
       'x-test-header': ['value1', 'value2'],
     };
     headers = new FastBootHeaders(headers);
-    expect(Ember.get(headers, 'x-test-header')).to.eq(headers.get('x-test-header'));
+
+    function fakeEmberGet(object, path) {
+      if (path in object) {
+        return object[path];
+      }
+
+      if ('unknownProperty' in object) {
+        return object.unknownProperty(path);
+      }
+
+      return undefined;
+    }
+
+    expect(fakeEmberGet(headers, 'x-test-header')).to.eq(headers.get('x-test-header'));
   });
 });