assert: respect assert.doesNotThrow message.

Special handling to detect when user has supplied a custom message.
Added a test for user message.
When testing if `actual` value is an error use
`util.isError` instead of `instanceof`.
Fixes: #2385
PR-URL: #2407
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Ilya Shaisultanov

lib/assert.js

@@ -330,7 +330,14 @@ function _throws(shouldThrow, block, expected, message) {
 fail(actual,expected,'Missing expected exception'+message);
 }
 
-if(!shouldThrow&&expectedException(actual,expected)){
+constuserProvidedMessage=typeofmessage==='string';
+constisUnwantedException=!shouldThrow&&util.isError(actual);
+constisUnexpectedException=!shouldThrow&&actual&&!expected;
+
+if((isUnwantedException&&
+userProvidedMessage&&
+expectedException(actual,expected))||
+isUnexpectedException){
 fail(actual,expected,'Got unwanted exception'+message);
 }
 

test/parallel/test-assert.js

@@ -321,6 +321,11 @@ assert.throws(function() {assert.ifError(new Error('test error'));});
 assert.doesNotThrow(function(){assert.ifError(null);});
 assert.doesNotThrow(function(){assert.ifError();});
 
+assert.throws(()=>{
+assert.doesNotThrow(makeBlock(thrower,Error),'user message');
+},/Gotunwantedexception.usermessage/,
+'a.doesNotThrow ignores user message');
+
 // make sure that validating using constructor really works
 threw=false;
 try{