chore: add eslint-plugin-compat and eslint-plugin-es5

- tests are not compiled with babel and must thus be in
  es5
- we don't use polyfills, so we need to make sure no
  functions aren't used that are not supported by popular browsers. (like Object.assign in Safari and IE11)
- Boths are errors that usually only appear when running
  tests in SauceLabs, which happens only on _after_
  merging a PR.

.eslintrc.js

@@ -1,5 +1,5 @@
 module.exports = {
-  "extends": "eslint:recommended",
+  "extends": ["eslint:recommended","plugin:compat/recommended"],
   "globals": {
     "self": false
   },

package.json

@@ -21,6 +21,7 @@
     "node": ">=0.4.7"
   },
   "dependencies": {
+    "eslint-plugin-compat": "^3.3.0",
     "neo-async": "^2.6.0",
     "optimist": "^0.6.1",
     "source-map": "^0.6.1"
@@ -37,6 +38,7 @@
     "dtslint": "^0.5.5",
     "dustjs-linkedin": "^2.0.2",
     "eco": "~1.1.0-rc-3",
+    "eslint-plugin-es5": "^1.4.1",
     "grunt": "^1.0.3",
     "grunt-babel": "^5.0.0",
     "grunt-bg-shell": "^2.3.3",

spec/.eslintrc

@@ -1,14 +1,19 @@
 {
+  "extends": [
+    "../.eslintrc.js",
+    "plugin:es5/no-es2015"
+  ],
+  "plugins": [
+    "es5"
+  ],
   "globals": {
     "CompilerContext": true,
     "Handlebars": true,
     "handlebarsEnv": true,
-
     "shouldCompileTo": true,
     "shouldCompileToWithPartials": true,
     "shouldThrow": true,
     "compileWithPartials": true,
-
     "console": true,
     "require": true,
     "suite": true,

spec/ast.js

@@ -126,7 +126,7 @@ describe('ast', function() {
   describe('whitespace control', function() {
     describe('parse', function() {
       it('mustache', function() {
-        let ast = Handlebars.parse('  {{~comment~}} ');
+        var ast = Handlebars.parse('  {{~comment~}} ');
 
         equals(ast.body[0].value, '');
         equals(ast.body[2].value, '');
@@ -142,7 +142,7 @@ describe('ast', function() {
 
     describe('parseWithoutProcessing', function() {
       it('mustache', function() {
-        let ast = Handlebars.parseWithoutProcessing('  {{~comment~}} ');
+        var ast = Handlebars.parseWithoutProcessing('  {{~comment~}} ');
 
         equals(ast.body[0].value, '  ');
         equals(ast.body[2].value, ' ');

spec/env/browser.js

@@ -9,7 +9,7 @@ var filename = 'dist/handlebars.js';
 if (global.minimizedTest) {
   filename = 'dist/handlebars.min.js';
 }
-var distHandlebars = fs.readFileSync(require.resolve(`../../${filename}`), 'utf-8');
+var distHandlebars = fs.readFileSync(require.resolve('../../' + filename), 'utf-8');
 vm.runInThisContext(distHandlebars, filename);
 
 global.CompilerContext = {