Find diagnostic locations for use in new compiler API.

dart/frog/tests/leg/src/mock_compiler.dart

Index: dart/frog/tests/leg/src/mock_compiler.dart
diff --git a/dart/frog/tests/leg/src/mock_compiler.dart b/dart/frog/tests/leg/src/mock_compiler.dart
index f1f86a367ced5ae8739d90603aa524771822fd7b..4203c91a5f966bb6b9b4faa270854dc382efd969 100644
--- a/dart/frog/tests/leg/src/mock_compiler.dart
+++ b/dart/frog/tests/leg/src/mock_compiler.dart
@@ -75,6 +75,10 @@ class MockCompiler extends Compiler {
     errors.add(new WarningMessage(node, message.message));
   }
 
+  void reportDiagnostic(SourceSpan span, String message, bool fatal) {
+    print(message);
+  }
+
   void clearWarnings() {
     warnings = [];
   }