util: show External values explicitly in inspect

Display `v8::External` values as `[External]` rather than `{}`
which makes them look like objects.

PR-URL: #12151
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: addaleax

lib/util.js

@@ -466,6 +466,9 @@ function formatValue(ctx, value, recurseTimes) {
       return `${constructor.name}` +
              ` { byteLength: ${formatNumber(ctx, value.byteLength)} }`;
     }
+    if (binding.isExternal(value)) {
+      return ctx.stylize('[External]', 'special');
+    }
   }
 
   var base = '';

src/node_util.cc

@@ -22,6 +22,7 @@ using v8::Value;
   V(isArrayBuffer, IsArrayBuffer)                                             \
   V(isDataView, IsDataView)                                                   \
   V(isDate, IsDate)                                                           \
+  V(isExternal, IsExternal)                                                   \
   V(isMap, IsMap)                                                             \
   V(isMapIterator, IsMapIterator)                                             \
   V(isPromise, IsPromise)                                                     \

test/parallel/test-util-inspect.js

@@ -82,6 +82,9 @@ assert.strictEqual(util.inspect(Object.assign(new String('hello'),
                    { [Symbol('foo')]: 123 }), { showHidden: true }),
                    '{ [String: \'hello\'] [length]: 5, [Symbol(foo)]: 123 }');
 
+assert.strictEqual(util.inspect(process.stdin._handle._externalStream),
+                   '[External]');
+
 {
   const regexp = /regexp/;
   regexp.aprop = 42;