Report error on only-try block.

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 41d6162645bdf31b5f5fbf3c948b8b3ebe134074..9c62894d42d5cc164dba6ad98ac3f16d4b420e4c 100644
--- a/pkg/front_end/tool/_fasta/generate_messages.dart
+++ b/pkg/front_end/tool/_fasta/generate_messages.dart
@@ -64,7 +64,7 @@ String compileTemplate(String name, String template, String tip) {
   var parameters = new Set<String>();
   var conversions = new Set<String>();
   var arguments = new Set<String>();
-  for (Match match in placeholderPattern.allMatches("$template$tip")) {
+  for (Match match in placeholderPattern.allMatches("$template${tip ?? ''}")) {
     switch (match[0]) {
       case "#character":
         parameters.add("String character");
@@ -93,6 +93,9 @@ String compileTemplate(String name, String template, String tip) {
         parameters.add("String string");
         arguments.add("'string': string,");
         break;
+
+      default:
+        throw "Unhandled placeholder in template: ${match[0]}";
     }
   }