Various infrastructure changes.

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 595f6f8620bcde3ab34e253f191b7a08fa6cdd59..596df086c2918d8d393b9248f8223ac95acd63bd 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -413,3 +413,16 @@ dynamic _classIdEqualsAssert(
 
   return instance;
 }
+
+/// Used by Fasta to report a runtime error when a final field with an
+/// initializer is also initialized in a generative constructor.
+///
+/// Note: in strong mode, this is a compile-time error and this class becomes
+/// obsolete.
+class _DuplicatedFieldInitializerError extends Error {
+  final String _name;
+
+  _DuplicatedFieldInitializerError(this._name);
+
+  toString() => "Error: field '$_name' is already initialized.";
+}