Babel setup for async/await (#1692)

* adds babel support for async/await with transpilation to ES2015 generators for backward compatibility with node 6 + working example of function converted to async/await syntax in linter.js

* rewrites run, scan, scanFile, getAddonMetadata and extractMetadata functions

* refactors the rest of the promises in linter.js

* fixes comments from second of review

* reverts unnecessary chahges from third round of review

.babelrc

@@ -1,7 +1,19 @@
 {
-  "presets": ["es2015", "stage-2"],
+  "presets": [
+    "es2015",
+    "stage-2"
+  ],
   "plugins": [
     "transform-class-properties",
     "transform-es2015-modules-commonjs",
+    "syntax-async-functions",
+    "transform-async-to-generator",
+    [
+      "transform-runtime",
+      {
+        "polyfill": false,
+        "regenerator": true
+      }
+    ]
   ]
 }

.eslintrc

@@ -2,7 +2,7 @@
   "rules":{
     "no-console": 0,
     // Inherited from airbnb via eslint-config-amo
-    'function-paren-newline': 0,
+    "function-paren-newline": 0
   },
   "env": {
     "node": true,
@@ -11,7 +11,12 @@
   },
   "extends": "amo",
   "parser": "babel-eslint",
+  "plugins": [
+    "async-await"
+  ],
   "settings": {
+    "async-await/space-after-async": 2,
+    "async-await/space-after-await": 2,
     "import/resolver": {
       "node": {
         // This adds ./src for relative imports.

package.json

@@ -35,17 +35,22 @@
     "babel-eslint": "8.0.2",
     "babel-jest": "22.0.0",
     "babel-loader": "7.1.2",
+    "babel-plugin-syntax-async-functions": "6.13.0",
+    "babel-plugin-transform-async-to-generator": "6.24.1",
     "babel-plugin-transform-class-properties": "6.24.1",
     "babel-plugin-transform-es2015-modules-commonjs": "6.26.0",
+    "babel-plugin-transform-runtime": "6.23.0",
     "babel-preset-env": "1.6.1",
     "babel-preset-es2015": "6.24.1",
     "babel-preset-stage-2": "6.24.1",
+    "babel-runtime": "6.26.0",
     "comment-json": "1.1.3",
     "coveralls": "3.0.0",
     "deepmerge": "2.0.1",
     "eslint-config-amo": "1.0.0",
+    "eslint-plugin-async-await": "0.0.0",
     "eslint-plugin-jest": "21.3.2",
-    "eslint-plugin-promise": "^3.5.0",
+    "eslint-plugin-promise": "3.5.0",
     "gfm.css": "1.1.1",
     "grunt": "1.0.1",
     "grunt-contrib-clean": "1.1.0",
@@ -57,7 +62,7 @@
     "is-mergeable-object": "1.1.0",
     "isparta": "4.0.0",
     "jest": "22.0.1",
-    "jest-raw-loader": "^1.0.1",
+    "jest-raw-loader": "1.0.1",
     "json-loader": "0.5.7",
     "load-grunt-configs": "1.0.0",
     "load-grunt-tasks": "3.5.2",
@@ -66,7 +71,7 @@
     "markdown-it": "8.4.0",
     "markdown-it-anchor": "4.0.0",
     "markdown-it-emoji": "1.4.0",
-    "raw-loader": "^0.5.1",
+    "raw-loader": "0.5.1",
     "request": "2.83.0",
     "sinon": "4.1.3",
     "tar": "4.1.0",
@@ -88,9 +93,9 @@
     "eslint-plugin-no-unsafe-innerhtml": "1.0.16",
     "esprima": "3.1.3",
     "first-chunk-stream": "2.0.0",
-    "fluent": "^0.4.1",
+    "fluent": "0.4.1",
     "jed": "1.1.1",
-    "pino": "^4.6.0",
+    "pino": "4.6.0",
     "postcss": "6.0.14",
     "probe-image-size": "3.2.0",
     "relaxed-json": "1.0.1",