Add test coverage with istanbul ...

* Add npm script `cover`, which calls istanbul:
  * Do not execute `istanbul` via `babel-node`, this is deprecated, instead use `babel-register` as an argument to `mocha` (credit: gotwarlost/istanbul#594 (comment))
  * Use `--root src/` to fix issue where istanbul also attempts to cover `mocha` (credit: douglasduteil/isparta#68 (comment))
  * Use `--include-all-sources` to make the coverage percentages actually meaningful by including sources that didn't get executed

* In `src/higher_order_components/errors_normalizer.js`, make minor change to return statement. This form: `if (foo) return { a: 'b' }` seems to have confused Istanbul's coverage highlighter. There are other cases like this, that are bugs in Istanbul (e.g. gotwarlost/istanbul#557). Probably worth making a repro case for this and reporting to them.

.gitignore

@@ -7,3 +7,5 @@ bower_components
 
 npm-debug.log
 .DS_Store
+
+coverage

package.json

@@ -12,6 +12,7 @@
     "test:watch": "npm run test -- --watch --watch-extensions js",
     "lint": "eslint src/ test/ || true",
     "lint:failfast": "eslint src/ test/",
+    "cover": "istanbul cover --root src/ --include-all-sources --report lcov --report json --report text --report html _mocha -- -r babel-register -r test/test_helper.js 'test/**/*.test.js'",
     "preversion": "./scripts/preversion.sh",
     "version": "./scripts/version.sh",
     "postversion": "./scripts/publish_docs.sh && git push && git push --tags"
@@ -28,6 +29,7 @@
     "babel-preset-es2015": "6.6.0",
     "babel-preset-react": "6.5.0",
     "babel-preset-stage-1": "6.5.0",
+    "babel-register": "^6.7.2",
     "chai": "3.5.0",
     "chai-enzyme": "^0.4.1",
     "dirty-chai": "^1.2.2",
@@ -37,13 +39,14 @@
     "eslint-plugin-react": "4.2.3",
     "extract-text-webpack-plugin": "1.0.1",
     "fs-extra": "0.26.7",
+    "istanbul": "^1.0.0-alpha.2",
     "jsdom": "8.2.0",
     "jsx-loader": "0.13.2",
     "mkpath": "1.0.0",
     "mocha": "2.4.5",
     "react": ">=0.14.3",
-    "react-dom": ">=0.14.3",
     "react-addons-test-utils": "^0.14.8",
+    "react-dom": ">=0.14.3",
     "webpack": "1.12.14",
     "webpack-dev-server": "1.14.1"
   },

src/higher_order_components/errors_normalizer.js

@@ -18,7 +18,9 @@ export default class ErrorNormalizer extends React.Component {
     // If errors is an array then convert it into an object. 'base' is
     // used to store all top-level errors that are not specific to an
     // input.
-    if (isArray) return { base: errors }
+    if (isArray) {
+      return { base: errors }
+    }
 
     return errors
   }