Fix some corner cases of inferring type parameters from bounds.

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 6613 matching lines @@
 
   /**
    * Get the [UnlinkedTypeParam]s representing the type parameters declared by
    * this element.
    *
    * TODO(scheglov) make private after switching linker to Impl
    */
   List<UnlinkedTypeParam> get unlinkedTypeParams;
 
   /**
+   * Determine the default value of type argument [i]. in most cases this will
+   * be `dynamic`, but sometimes it will be the bound of the ith type parameter.
+   */
+  DartType computeDefaultTypeArgument(int i) {
+    // If strong mode is off, or we can tell quickly from the summary that there
+    // is no bound, then the default type argument is `dynamic`; we don't have
+    // to call `typeParameters` to find that out.
+    if (!context.analysisOptions.strongMode ||
+        (unlinkedTypeParams != null && unlinkedTypeParams[i].bound == null)) {
+      return DynamicTypeImpl.instance;
+    }
+    return typeParameters[i].bound ?? DynamicTypeImpl.instance;
+  }
+
+  /**
    * Convert the given [index] into a type parameter type.
    */
   TypeParameterType getTypeParameterType(int index) {
     List<TypeParameterType> types = typeParameterTypes;
     if (index <= types.length) {
       return types[types.length - index];
     } else if (enclosingTypeParameterContext != null) {
       return enclosingTypeParameterContext
           .getTypeParameterType(index - types.length);
     } else {
@@ skipping 231 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }