Fix some corner cases of inferring type parameters from bounds.

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

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 923ae2ca96fe211c82c29282d60c6d9bedbc04f5..d46c864189b0b3cf1b6516337cd738da05af5e83 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -6631,6 +6631,21 @@ abstract class TypeParameterizedElementMixin
   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) {