Add --set-exit-code-on-problem option.

pkg/front_end/lib/src/fasta/messages.dart

Index: pkg/front_end/lib/src/fasta/messages.dart
diff --git a/pkg/front_end/lib/src/fasta/messages.dart b/pkg/front_end/lib/src/fasta/messages.dart
index 5f3f7675370c1275bfff116bfe65e58df400d0d9..7224d978247272a868da5c5baffa89d8af592266 100644
--- a/pkg/front_end/lib/src/fasta/messages.dart
+++ b/pkg/front_end/lib/src/fasta/messages.dart
@@ -4,6 +4,8 @@
 
 library fasta.messages;
 
+import 'dart:io' show exitCode;
+
 import 'package:kernel/ast.dart' show Library, Location, Program, TreeNode;
 
 import 'util/relativize.dart' show relativizeUri;
@@ -30,6 +32,10 @@ bool get isVerbose => CompilerContext.current.options.verbose;
 
 bool get hideNits => !isVerbose;
 
+bool get setExitCodeOnProblem {
+  return CompilerContext.current.options.setExitCodeOnProblem;
+}
+
 void warning(Message message, int charOffset, Uri uri) {
   if (hideWarnings) return;
   print(deprecated_format(
@@ -85,6 +91,9 @@ String colorNit(String message) {
 }
 
 String deprecated_format(Uri uri, int charOffset, String message) {
+  if (setExitCodeOnProblem) {

[Siggi Cherem (dart-lang), 2017/07/11 17:04:16]

just to make sure, we would practically only get here  when verbose is true,
correct?

[ahe, 2017/07/11 17:52:36]

No. We get here if we print any diagnostic. But I think that's what we want: the
exit code is set to 1 if we print an error, a warning, or a nit.

[Siggi Cherem (dart-lang), 2017/07/11 18:12:34]

OK - so it is done only if something is visible in stdout (I just wanted to make
sure we don't set the exit code and provide no indication of it).

Eventually I'd like to do two changes to this:
 (a) collect all print statements in a single place (since that'll be done only
when there is no listener for messages in the public APIs).
 (b) set the exit code only when we are handling errors ourselves. If someone
wants to handle errors by setting their listener, then they can also handle the
exit code themselves.

I'm OK doing this latter as a follow up, in which case please add a TODO for me
here.

[ahe, 2017/07/12 09:59:06]

I have already implemented this, and should be able to send out a CL for it
later today, so I'll just skip the TODO.

+    exitCode = 1;
+  }
   if (uri != null) {
     String path = relativizeUri(uri);
     Location location = charOffset == -1 ? null : getLocation(path, charOffset);