Fix: code coverage

Replace Istanbul with nyc, Istanbul's CLI. nyc appears to have some
bugs that this patch works around:

  - When all files in the project are considered, not just those
    imported via tests, the coverage rates and line numbers vary between
    runs. This patch disables the `all` option for now and points to the
    bug:
    https://github.com/istanbuljs/nyc/issues/537#issuecomment-390814662.

  - Source map line numbers appear to be incorrect except when `all` is
    enabled and working correctly (see previous bullet).

  - `sourceMap` must be disabled to avoid ENAMETOOLONG errors when nyc
    tries to include them as encoded strings. The patch disables the
    setting and points to: https://github.com/istanbuljs/nyc/issues/847.
    Using babel-plugin-istanbul and source-map-support appears to have
    no effect (the former in tests/node-qunit/run.js and .babelrc).

  - CI fails with
    `Error: EACCES: permission denied, mkdir '/nonexistent'`. Specify
    `SPAWN_WRAP_SHIM_ROOT` instead of constructing a subdirectory from a
    nonexistent home directory.

Bug: T196952
Bug: T193519
Change-Id: Idf2e3accd4a6277cbef91c1156fcd206c9e7d882

.gitignore

@@ -1,5 +1,5 @@
 /doc/autogenerated/
-/coverage/
+/.nyc_output/
 /node_modules/
 /vendor/
 /composer.lock

.nycrc.json

@@ -0,0 +1,31 @@
+{
+	"//": "todo: enable caching. There are too many other issues to verify that",
+	"//": "caching works correctly.",
+	"cache": false,
+
+	"//": "todo: check coverage on all files not just those included in tests.",
+	"//": "Enable when",
+	"//": "https://github.com/istanbuljs/nyc/issues/537#issuecomment-390814662",
+	"//": "is fixed.",
+	"all": false,
+
+	"//": "Ignore files in vendor/, resources/, and elsewhere.",
+	"include": [ "src/**/*.js" ],
+
+	"//": "Set the coverage percentage by category thresholds.",
+	"statements": 90,
+	"branches": 85,
+	"functions": 85,
+	"lines": 90,
+
+	"//": "Fail if the coverage is below threshold.",
+	"check-coverage": true,
+
+	"//": "Work around source maps being included ",
+	"//": "https://github.com/istanbuljs/nyc/issues/847:",
+	"//":  "  Error: ENAMETOOLONG: name too long, open '.../vagrant/mediawiki/extensions/Popups/src/data:application/json;...'",
+	"//": "Unfortunately, the reported line numbers appear to be",
+	"//": "nondeterministic across runs when all is enabled and incorrect when",
+	"//": "disabled.",
+	"sourceMap": false
+}

package.json

@@ -5,13 +5,13 @@
     "build": "NODE_ENV=production webpack -p",
     "lint:fix": "grunt fix",
     "test:node": "node tests/node-qunit/run.js 'tests/node-qunit/**/*.test.js' | tap-dot",
-    "test:dev": "grunt lint && npm run test:node",
-    "test": "npm run check-built-assets && npm run test:dev && npm run doc",
+    "test:dev": "grunt lint && npm -s run coverage && npm -s run -s doc",
+    "test": "npm -s run check-built-assets && npm -s run test:dev",
     "doc": "jsdoc -c jsdoc.json",
-    "doc:start": "nodemon --watch src --exec 'npm run doc'",
-    "check-built-assets": "echo 'CHECKING BUILD SOURCES ARE COMMITTED OR STAGED' && npm run build && git diff -q resources/dist",
-    "coverage": "istanbul cover node_modules/.bin/mw-node-qunit tests/node-qunit/*.js tests/node-qunit/**/*.js",
-    "precommit": "npm run test:dev && npm run build && git add resources/dist && bundlesize"
+    "doc:start": "nodemon --watch src --exec 'npm -s run doc'",
+    "check-built-assets": "echo 'CHECKING BUILD SOURCES ARE COMMITTED OR STAGED' && npm -s run build && git diff -q resources/dist",
+    "coverage": "SPAWN_WRAP_SHIM_ROOT=\"$PWD/.nyc_output\" nyc npm -s run test:node",
+    "precommit": "npm -s run test:dev && npm -s run build && git add resources/dist && bundlesize"
   },
   "devDependencies": {
     "babel-loader": "7.1.4",
@@ -27,10 +27,10 @@
     "grunt-jsonlint": "1.1.0",
     "grunt-stylelint": "0.10.0",
     "husky": "0.13.3",
-    "istanbul": "0.4.5",
     "jsdoc": "3.5.5",
     "mw-node-qunit": "4.0.0",
     "nodemon": "1.17.3",
+    "nyc": "12.0.2",
     "redux": "3.6.0",
     "redux-thunk": "2.2.0",
     "stylelint": "9.2.0",