Add code coverage. (GoogleChrome#2637)

* Node coverage working.

* Get karma to play nice with code coverage.

* Get config for nyc/istanbul working.

Author: robdodson

.nycrc

@@ -0,0 +1,13 @@
+{
+  "include": [
+    "src/**/*.js",
+    "shared/**/*.js",
+    "./*.js"
+  ],
+  "exclude": [
+    "node_modules/**/*.js",
+    "test/**/*.js",
+    ".eleventy.js",
+    "karma.conf.js"
+  ]
+}

build.js

@@ -25,6 +25,7 @@ const rollupPluginCJS = require('rollup-plugin-commonjs');
 const rollupPluginPostCSS = require('rollup-plugin-postcss');
 const rollupPluginVirtual = require('rollup-plugin-virtual');
 const rollupPluginReplace = require('rollup-plugin-replace');
+const rollupPluginIstanbul = require('rollup-plugin-istanbul');
 const OMT = require('@surma/rollup-plugin-off-main-thread');
 const rollup = require('rollup');
 const terser = isProd ? require('terser') : null;
@@ -230,6 +231,7 @@ async function buildTest() {
       rollupPluginCJS(),
       rollupPluginVirtual(buildVirtualJSON(virtualImports)),
       rollupPluginPostCSS(),
+      rollupPluginIstanbul(),
     ],
   });
   await testBundle.write({