refactor(test/prefer-native): use co instead of async fn

package.json

@@ -37,6 +37,7 @@
     "broccoli-asset-rev": "^2.4.5",
     "broccoli-test-helper": "^1.2.0",
     "chai": "^4.1.2",
+    "co": "^4.6.0",
     "ember-cli": "~3.0.2",
     "ember-cli-dependency-checker": "^3.0.0",
     "ember-cli-eslint": "^4.2.1",

test/prefer-native-test.js

@@ -6,6 +6,7 @@ const helpers = require('broccoli-test-helper');
 const vm = require('vm');
 const fs = require('fs');
 const RSVP = require('rsvp');
+const co = require('co');
 
 const testCode = `
 define('fetch-test', ['fetch'], function(_fetch) {
@@ -29,21 +30,21 @@ require('fetch-test');
       output = helpers.createBuilder(subject);
     });
 
-    afterEach(async function() {
-      await output.dispose();
-    });
+    afterEach(co.wrap(function* () {
+      yield output.dispose();
+    }));
 
-    it('preferNative is built into vendor file', async function() {
-      await output.build();
+    it('preferNative is built into vendor file', co.wrap(function*() {
+      yield output.build();
       let files = output.read();
       expect(files).to.have.all.keys('ember-fetch.js');
       expect(files['ember-fetch.js']).to.include(`var preferNative = ${preferNative}`);
-    });
+    }));
 
     it(`${
       preferNative ? 'Prefers' : "Doesn't prefer"
-    } native fetch as specified`, async function() {
-      await output.build();
+    } native fetch as specified`, co.wrap(function*() {
+      yield output.build();
       let emberFetchCode = output.read()['ember-fetch.js'];
       const amdLoaderCode = fs.readFileSync(require.resolve('loader.js'));
       const sandbox = {
@@ -59,11 +60,11 @@ require('fetch-test');
       const code = amdLoaderCode + emberFetchCode + testCode;
       vm.runInContext(code, sandbox);
       expect(sandbox.__result).to.equal(preferNative);
-    });
+    }));
 
     if (preferNative === true) {
-      it('Has fetch poly fill even if fetch is not there', async function() {
-        await output.build();
+      it('Has fetch poly fill even if fetch is not there', co.wrap(function*() {
+        yield output.build();
         let emberFetchCode = output.read()['ember-fetch.js'];
         const amdLoaderCode = fs.readFileSync(require.resolve('loader.js'));
         const sandbox = {
@@ -87,7 +88,7 @@ require('fetch-test');
         const code = amdLoaderCode + emberFetchCode + testCodeForNonFetch;
         vm.runInContext(code, sandbox);
         expect(sandbox.window.__result).to.equal(true);
-      })
+      }))
     }
   });
 });