Update ConstructorNamedArgumentTest to use the 'runtime-error' directive

tools/testing/dart/multitest.dart

Index: tools/testing/dart/multitest.dart
diff --git a/tools/testing/dart/multitest.dart b/tools/testing/dart/multitest.dart
index 64f09ce789ed3c6bd6929cf5428178dc34b1a99f..a637c987e23872ae08b9ca0b87ac5a08127bd96f 100644
--- a/tools/testing/dart/multitest.dart
+++ b/tools/testing/dart/multitest.dart
@@ -81,7 +81,9 @@ void ExtractTestsFromMultitest(String filename,
   // except dart: or /, at the beginning of a line.
   RegExp relativeImportRegExp =
       const RegExp('^#(import|source)[(]["\'](?!(dart:|/))');
+  int lineCount = 0;
   for (String line in lines) {
+    lineCount++;
     if (line.contains('///')) {
       var parts = line.split('///')[1].split(':');
       var key = parts[0].trim();
@@ -92,7 +94,9 @@ void ExtractTestsFromMultitest(String filename,
       } else {
         (testsAsLines[key] = new List<String>.from(testTemplate)).add(line);
         outcomes[key] = rest;
-        Expect.isTrue(validMultitestOutcomes.contains(rest));
+        if (!validMultitestOutcomes.contains(rest)) {
+          Expect.fail("Invalid test directive on line ${lineCount}: $rest ");
+        }
       }
     } else {
       testTemplate.add(line);