Reinsert (and test) handling of exceptions from user code.

pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart

Index: pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart
diff --git a/pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart b/pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart
index 3cb959fbfda533a91c1642bfd8cde2557d7569a4..e31edde9fff1f47a915207d4126da47c35ea7a69 100644
--- a/pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart
+++ b/pkg/compiler/lib/src/diagnostics/diagnostic_listener.dart
@@ -60,6 +60,10 @@ abstract class DiagnosticReporter {
 
   DiagnosticMessage createMessage(Spannable spannable, MessageKind messageKind,
       [Map arguments = const {}]);
+
+  /// Called when an [exception] is thrown from user-provided code, like from
+  /// the input provider or diagnostics handler.
+  void onCrashInUserCode(String message, exception, stackTrace) {}
 }
 
 class DiagnosticMessage {