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 6915d4fae03cabce6880b18e415676b68fb17db4..e04d087148edbc25716187982fde90c8bf129c7d 100644
--- a/pkg/front_end/tool/_fasta/generate_messages.dart
+++ b/pkg/front_end/tool/_fasta/generate_messages.dart
@@ -51,6 +51,10 @@ 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('missing template for message: $name');
+    return '';

[Paul Berry, 2017/07/11 16:00:59]

Would it be better to throw an exception here, to make sure that the developer
doesn't accidentally miss the warning message and assume compilation has
succeeded?

[Siggi Cherem (dart-lang), 2017/07/12 00:29:46]

Done.

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