[frontend_server] Don't print Severity.info messages

The info severity level has only been used in one instance before, and
that could only occur in the compile command.

https://dart-review.googlesource.com/c/sdk/+/180361 and
https://dart-review.googlesource.com/c/sdk/+/180560 introduced the first
uses of this severity level outside the compile command.
This is the first time an `info` is emitted in the frontend_server.

We don't want to emit `info`s for the server. This is a quick solution
to fix that. The solution has been manually verified with a Flutter app
run with a local engine build with this change patched in.
Bug: flutter/flutter#75429

Keeping the diff as small as possible to ease cherry pick for release.

The better solution is to make the frontend_server's verbosity level
configurable.
Bug: #44867

Change-Id: Ie40dbb27621053e90bc8943b8cb7c5ec4c9ed643
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183005
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Daco Harkes <[email protected]>

Author: dcharkes

pkg/frontend_server/lib/frontend_server.dart

@@ -353,6 +353,8 @@ class FrontendCompiler implements CompilerInterface {
   final List<String> errors = <String>[];
 
   _onDiagnostic(DiagnosticMessage message) {
+    // TODO(https://dartbug.com/44867): The frontend server should take a
+    // verbosity argument and put that in CompilerOptions and use it here.
     bool printMessage;
     switch (message.severity) {
       case Severity.error:
@@ -361,9 +363,11 @@ class FrontendCompiler implements CompilerInterface {
         errors.addAll(message.plainTextFormatted);
         break;
       case Severity.warning:
-      case Severity.info:
         printMessage = true;
         break;
+      case Severity.info:
+        printMessage = false;
+        break;
       case Severity.context:
       case Severity.ignored:
         throw 'Unexpected severity: ${message.severity}';