Add optional message to assert in Dart2js - continued

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index 8fabdf8e91de92285ee11fdaf233fa78b8fb7efa..f2911585d5b3067de168eb7006480f1acecf7b6a 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -188,6 +188,7 @@ enum MessageKind {
   EXISTING_DEFINITION,
   EXISTING_LABEL,
   EXPECTED_IDENTIFIER_NOT_RESERVED_WORD,
+  EXPERIMENTAL_ASSERT_MESSAGE,
   EXPONENT_MISSING,
   EXPORT_BEFORE_PARTS,
   EXTERNAL_WITH_BODY,
@@ -3269,6 +3270,19 @@ main() => foo();
   // Patch errors end.
   //////////////////////////////////////////////////////////////////////////////
 
+      MessageKind.EXPERIMENTAL_ASSERT_MESSAGE:
+        const MessageTemplate(MessageKind.EXPERIMENTAL_ASSERT_MESSAGE,
+          "Experimental language feature 'assertion with message'"
+          " is not supported.",
+          howToFix:
+            "Use option '--assert-message' to use assertions with messages.",
+          examples: const [r'''
+main() {
+  int n = -7;
+  assert(n > 0, 'must be positive: $n');
+}
+''']),
+
       MessageKind.IMPORT_EXPERIMENTAL_MIRRORS:
         const MessageTemplate(MessageKind.IMPORT_EXPERIMENTAL_MIRRORS, r'''