Add assertion on addImplementationBuilder.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.source_library_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../combinator.dart' show Combinator;
 
@@ skipping 52 matching lines @@
   Uri partOfUri;
 
   List<MetadataBuilder> metadata;
 
   /// The current declaration that is being built. When we start parsing a
   /// declaration (class, method, and so on), we don't have enough information
   /// to create a builder and this object records its members and types until,
   /// for example, [addClass] is called.
   DeclarationBuilder<T> currentDeclaration;
 
+  bool canAddImplementationBuilders = false;
+
   SourceLibraryBuilder(this.loader, Uri fileUri)
       : fileUri = fileUri,
         super(fileUri) {
     currentDeclaration = libraryDeclaration;
   }
 
   Uri get uri;
 
   bool get isPart => partOfName != null || partOfUri != null;
 
@@ skipping 213 matching lines @@
       // extends Object with Mixin {}` in the same library.
       return !existing.isMixinApplication || !other.isMixinApplication;
     }
     return true;
   }
 
   void buildBuilder(Builder builder);
 
   R build() {
     assert(implementationBuilders.isEmpty);
+    canAddImplementationBuilders = true;
     forEach((String name, Builder builder) {
       do {
         buildBuilder(builder);
         builder = builder.next;
       } while (builder != null);
     });
     for (List list in implementationBuilders) {
       String name = list[0];
       Builder builder = list[1];
       int charOffset = list[2];
       addBuilder(name, builder, charOffset);
       buildBuilder(builder);
     }
+    canAddImplementationBuilders = false;
     return null;
   }
 
+  /// Used to add implementation builder during the call to [build] above.
+  /// Currently, only anonymous mixins are using implementation builders (see
+  /// [KernelMixinApplicationBuilder]
+  /// (../kernel/kernel_mixin_application_builder.dart)).
   void addImplementationBuilder(String name, Builder builder, int charOffset) {
+    assert(canAddImplementationBuilders, "$uri");
     implementationBuilders.add([name, builder, charOffset]);
   }
 
   void validatePart() {
     if (parts.isNotEmpty) {
       inputError(fileUri, -1,
           "A file that's a part of a library can't have parts itself.");
     }
     if (exporters.isNotEmpty) {
       Export export = exporters.first;
@@ skipping 204 matching lines @@
   }
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 }