assert: fix .throws operator

assert.throws and assert.doesNotThrow set the operator to a
internal function. That was by accident and originally the operator
was undefined. This changes it to show "throws" or "doesNotThrow".

Backport-PR-URL: #23223
PR-URL: #17575
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

lib/assert.js

@@ -643,15 +643,18 @@ function innerThrows(shouldThrow, block, expected, message) {
         details += ` (${expected.name})`;
       }
       details += message ? `: ${message}` : '.';
-      fail(actual, expected, `Missing expected exception${details}`, fail);
+      fail(actual, expected, `Missing expected exception${details}`, 'throws');
     }
     if (expected && expectedException(actual, expected) === false) {
       throw actual;
     }
   } else if (actual !== undefined) {
     if (!expected || expectedException(actual, expected)) {
       details = message ? `: ${message}` : '.';
-      fail(actual, expected, `Got unwanted exception${details}`, fail);
+      fail(actual,
+           expected,
+           `Got unwanted exception${details}`,
+           'doesNotThrow');
     }
     throw actual;
   }

test/parallel/test-assert.js

@@ -463,10 +463,15 @@ assert.throws(() => { assert.ifError(new Error('test error')); },
 assert.doesNotThrow(() => { assert.ifError(null); });
 assert.doesNotThrow(() => { assert.ifError(); });
 
-assert.throws(() => {
-  assert.doesNotThrow(makeBlock(thrower, Error), 'user message');
-}, /Got unwanted exception: user message/,
-              'a.doesNotThrow ignores user message');
+common.expectsError(
+  () => assert.doesNotThrow(makeBlock(thrower, Error), 'user message'),
+  {
+    type: a.AssertionError,
+    code: 'ERR_ASSERTION',
+    operator: 'doesNotThrow',
+    message: 'Got unwanted exception: user message\n[object Object]'
+  }
+);
 
 // make sure that validating using constructor really works
 {
@@ -525,7 +530,8 @@ a.throws(makeBlock(thrower, TypeError), (err) => {
     () => { a.throws((noop)); },
     common.expectsError({
       code: 'ERR_ASSERTION',
-      message: /^Missing expected exception\.$/
+      message: /^Missing expected exception\.$/,
+      operator: 'throws'
     }));
 
   assert.throws(