Start marking @_fastaProblem, fix for unresolved mixin, fix for type parameter context.

pkg/analyzer/lib/src/dart/element/element.dart

 // Copyright (c) 2014, 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 analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1247 matching lines @@
     if (_kernelSupertype == null) {
       _kernelMixins = <kernel.Supertype>[];
       kernel.Supertype supertype = _kernel.supertype;
       if (supertype != null) {
         if (_kernel.mixedInType != null) {
           _kernelMixins.add(_kernel.mixedInType);
         }
         while (supertype.classNode.isSyntheticMixinImplementation) {
           var superNode = supertype.classNode;
           var substitute = kernel.Substitution.fromSupertype(supertype);
-          var thisMixin = substitute.substituteSupertype(superNode.mixedInType);
-          _kernelMixins.add(thisMixin);
+
+          var superMixin = superNode.mixedInType;
+          if (superMixin != null) {
+            var thisMixin = substitute.substituteSupertype(superMixin);
+            _kernelMixins.add(thisMixin);
+          }
+
           supertype = substitute.substituteSupertype(superNode.supertype);
         }
         _kernelMixins = _kernelMixins.reversed.toList();
       }
       _kernelSupertype = supertype;
     }
   }
 
   /**
    * Resynthesize explicit fields and property accessors and fill [_fields] and
@@ skipping 8269 matching lines @@
 
   @override
   DartObject computeConstantValue() => null;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     super.visitChildren(visitor);
     _initializer?.accept(visitor);
   }
 }