Add a few more known non-null cases

test/codegen/expect/unittest/unittest.js

 dart_library.library('unittest/unittest', null, /* Imports */[
   'dart/_runtime',
   'matcher/matcher',
   'dom/dom',
   'dart/core',
   'dart/async',
   'matcher/src/interfaces',
   'matcher/src/util',
   'matcher/src/description'
 ], /* Lazy imports */[
@@ skipping 62 matching lines @@
   function _defaultFailFormatter(actual, matcher, reason, matchState, verbose) {
     let description = new description$.StringDescription();
     description.add('Expected: ').addDescriptionOf(matcher).add('\n');
     description.add('  Actual: ').addDescriptionOf(actual).add('\n');
     let mismatchDescription = new description$.StringDescription();
     matcher.describeMismatch(actual, mismatchDescription, matchState, verbose);
     if (dart.notNull(mismatchDescription.length) > 0) {
       description.add(`   Which: ${mismatchDescription}\n`);
     }
     if (reason != null) description.add(reason).add('\n');
-    return dart.toString(description);
+    return description.toString();
   }
   dart.fn(_defaultFailFormatter, core.String, [dart.dynamic, interfaces.Matcher, core.String, core.Map, core.bool]);
   // Exports:
   exports.group = group;
   exports.test = test;
   exports.TestFailure = TestFailure;
   exports.ErrorFormatter = ErrorFormatter;
   exports.expect = expect;
   exports.fail = fail;
 });