Throw AbstractClassInstantiationError on abstract classes.

pkg/front_end/lib/src/fasta/loader.dart

Index: pkg/front_end/lib/src/fasta/loader.dart
diff --git a/pkg/front_end/lib/src/fasta/loader.dart b/pkg/front_end/lib/src/fasta/loader.dart
index 0af3e1f31f5d33fa28e49a49ca28f417d1634f49..f8a7b4231de619a1ff8b9b7fa5a7b27336557101 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -139,6 +139,10 @@ ${format(ms / libraryCount, 3, 12)} ms/compilation unit.""");
   Builder getCompileTimeError() => target.getCompileTimeError(this);
 
   Builder getNativeAnnotation() => target.getNativeAnnotation(this);
+
+  Builder getAbstractClassInstantiationError() {
+    return target.getAbstractClassInstantiationError(this);
+  }
 }
 
 String format(double d, int fractionDigits, int width) {