Improvements to warnings on message extraction.

pkg/intl/test/message_extraction/failed_extraction_test.dart

+import "message_extraction_test.dart";
+import "dart:io";
+import "package:unittest/unittest.dart";
+
+main() {
+  run(null, ["extract_to_json.dart",
+      "sample_with_messages.dart", "part_of_sample_with_messages.dart"])
+    .then((ProcessResult result) {
+      expect(result.exitCode, 0);
+  });
+  run(null, ["extract_to_json.dart", "--warnings-are-errors",
+      "sample_with_messages.dart", "part_of_sample_with_messages.dart"])
+    .then((ProcessResult result) {
+      expect(result.exitCode, 1);
+    });
+}