Prepare for improved error recovery from compile-time errors.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
diff --git a/runtime/lib/errors_patch.dart b/runtime/lib/errors_patch.dart
index d4bf0e78f6fe8fce555d82eff0bb409c3f87159d..4690ba28a35a196f5f521784cde8a249d8b9c9ef 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -430,3 +430,9 @@ class _DuplicatedFieldInitializerError extends Error {
 
   toString() => "Error: field '$_name' is already initialized.";
 }
+
+@patch
+class _ConstantExpressionError {
+  @patch
+  _throw(error) => throw error;
+}