Complain about external methods with a body.

pkg/front_end/lib/src/fasta/builder/member_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/member_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/member_builder.dart b/pkg/front_end/lib/src/fasta/builder/member_builder.dart
index f21dff6da0f44ad0f979b1905297e28ded3f83a3..0838145468efdc6721f537d919e13fdcebae6677 100644
--- a/pkg/front_end/lib/src/fasta/builder/member_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/member_builder.dart
@@ -4,7 +4,8 @@
 
 library fasta.member_builder;
 
-import 'builder.dart' show Builder, ClassBuilder, ModifierBuilder;
+import 'builder.dart'
+    show Builder, ClassBuilder, LibraryBuilder, ModifierBuilder;
 
 abstract class MemberBuilder extends ModifierBuilder {
   /// For top-level members, the parent is set correctly during
@@ -28,6 +29,16 @@ abstract class MemberBuilder extends ModifierBuilder {
 
   bool get isRedirectingGenerativeConstructor => false;
 
+  LibraryBuilder get library {
+    if (parent is LibraryBuilder) {
+      LibraryBuilder library = parent;
+      return library.partOfLibrary ?? library;
+    } else {
+      ClassBuilder cls = parent;
+      return cls.library;
+    }
+  }
+
   @override
   String get fullNameForErrors => name;
 }