Prepare for improved error recovery from compile-time errors.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 082809564f7b7e39dd6ce535a97ec9a865bdb3db..1fd040e8fd65bab027d91cfd29c914405578b160 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -517,4 +517,13 @@ class SourceLoader<L> extends Loader<L> {
         isConstructor: isConstructor,
         isTopLevel: isTopLevel);
   }
+
+  Expression throwCompileConstantError(Expression error) {
+    return target.backendTarget.throwCompileConstantError(coreTypes, error);
+  }
+
+  Expression buildCompileTimeError(String message, int offset) {
+    return target.backendTarget
+        .buildCompileTimeError(coreTypes, message, offset);
+  }
 }