feat: allow babel cache to be enabled (#517)

bin/nyc.js

@@ -1,8 +1,5 @@
 #!/usr/bin/env node
 
-// the babel cache does not play nicely with nyc.
-process.env.BABEL_DISABLE_CACHE = '1'
-
 var configUtil = require('../lib/config-util')
 var foreground = require('foreground-child')
 var NYC
@@ -50,9 +47,15 @@ if (argv._[0] === 'report') {
     NYC_CONFIG: JSON.stringify(config),
     NYC_CWD: process.cwd(),
     NYC_ROOT_ID: nyc.rootId,
-    BABEL_DISABLE_CACHE: 1,
     NYC_INSTRUMENTER: config.instrumenter
   }
+
+  if (config['babel-cache'] === false) {
+    // babel's cache interferes with some configurations, so is
+    // disabled by default. opt in by setting babel-cache=true.
+    env.BABEL_DISABLE_CACHE = process.env.BABEL_DISABLE_CACHE = '1'
+  }
+
   sw([wrapper], env)
 
   // Both running the test script invocation and the check-coverage run may

lib/config-util.js

@@ -142,6 +142,11 @@ Config.buildYargs = function (cwd) {
       type: 'boolean',
       describe: 'cache instrumentation results for improved performance'
     })
+    .option('babel-cache', {
+      default: false,
+      type: 'boolean',
+      describe: 'cache babel transpilation results for improved performance'
+    })
     .option('extension', {
       alias: 'e',
       default: [],