fix: fs.promises was not introduced until 10 (#67)

lib/v8-to-istanbul.js

@@ -5,9 +5,14 @@ const { dirname, join } = require('path')
 const CovBranch = require('./branch')
 const CovFunction = require('./function')
 const CovSource = require('./source')
-const { readFile } = require('fs').promises
+const compatError = Error(`requires Node.js ${require('../package.json').engines.node}`)
+let readFile = () => { throw compatError }
+try {
+  readFile = require('fs').promises.readFile
+} catch (_err) {
+  // most likely we're on an older version of Node.js.
+}
 const { SourceMapConsumer } = require('source-map')
-
 const isOlderNode10 = /^v10\.(([0-9]\.)|(1[0-5]\.))/u.test(process.version)
 
 // Injected when Node.js is loading script into isolate pre Node 10.16.x.