Use messages for (some) public API errors

pkg/front_end/tool/_fasta/generate_messages.dart

Index: pkg/front_end/tool/_fasta/generate_messages.dart
diff --git a/pkg/front_end/tool/_fasta/generate_messages.dart b/pkg/front_end/tool/_fasta/generate_messages.dart
index 2e37db788b88dbc4cfdbfbabf2be1b65e5bb25f0..4b7ecda6778fa88b4ec9ec4e3b890edd10cf696b 100644
--- a/pkg/front_end/tool/_fasta/generate_messages.dart
+++ b/pkg/front_end/tool/_fasta/generate_messages.dart
@@ -53,6 +53,11 @@ final RegExp placeholderPattern = new RegExp("#[a-zA-Z0-9_]+");
 
 String compileTemplate(String name, String template, String tip,
     String analyzerCode, String dart2jsCode) {
+  if (template == null) {
+    print('Error: missing template for message: $name');
+    exit(1);

[ahe, 2017/07/13 00:03:38]

How about:

exitCode = 1;

[Siggi Cherem (dart-lang), 2017/07/18 00:32:57]

Done.

+    return '';
+  }
   var parameters = new Set<String>();
   var conversions = new Set<String>();
   var arguments = new Set<String>();