Prepare for improved error recovery from compile-time errors.

sdk/lib/core/errors.dart

Index: sdk/lib/core/errors.dart
diff --git a/sdk/lib/core/errors.dart b/sdk/lib/core/errors.dart
index 4dfa006ca7cfba6daed3d5ec0e5e946e4255268a..e563990161df2168bc66ed5bd27bcb9a41c17908 100644
--- a/sdk/lib/core/errors.dart
+++ b/sdk/lib/core/errors.dart
@@ -557,3 +557,11 @@ class CyclicInitializationError extends Error {
       ? "Reading static variable during its initialization"
       : "Reading static variable '$variableName' during its initialization";
 }
+
+/// Used by Fasta to throw a compile-time error in a way that is compatible
+/// with compile-time constant evaluation.
+class _ConstantExpressionError {
+  const _ConstantExpressionError();
+
+  external _throw(error);
+}