Fix summarizing of types with unused type parameters.

pkg/analyzer/lib/src/summary/summarize_elements.dart

 // Copyright (c) 2015, 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 serialization.elements;
 
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 432 matching lines @@
     buildingLinkedReferences = true;
     linkedUnit.types = deferredLinkedTypes
         .map((_SerializeTypeRef closure) => closure())
         .toList();
     linkedUnit.constCycles = constCycles;
     buildingLinkedReferences = false;
   }
 
   /**
    * Compute the appropriate De Bruijn index to represent the given type
-   * parameter [type].
+   * parameter [type], or return `null` if the type parameter is not in scope.
    */
   int findTypeParameterIndex(TypeParameterType type, Element context) {
     Element originalContext = context;
     int index = 0;
     while (context != null) {
       List<TypeParameterElement> typeParameters;
       if (context is ClassElement) {
         typeParameters = context.typeParameters;
       } else if (context is FunctionTypeAliasElement) {
         typeParameters = context.typeParameters;
       } else if (context is ExecutableElement) {
         typeParameters = context.typeParameters;
       }
       if (typeParameters != null) {
         for (int i = 0; i < typeParameters.length; i++) {
           TypeParameterElement param = typeParameters[i];
           if (param == type.element) {
             return index + typeParameters.length - i;
           }
         }
         index += typeParameters.length;
       }
       context = context.enclosingElement;
     }
-    throw new StateError(
-        'Unbound type parameter $type (${originalContext?.location})');
+    return null;
   }
 
   /**
    * Get the type arguments for the given [type], or `null` if the type has no
    * type arguments.
    *
    * TODO(paulberry): consider adding an abstract getter to [DartType] to do
    * this.
    */
   List<DartType> getTypeArguments(DartType type) {
@@ skipping 512 matching lines @@
    * interpreted; this is used to serialize type parameters.
    */
   EntityRefBuilder serializeTypeRef(DartType type, Element context,
       {int slot}) {
     if (slot != null) {
       assert(buildingLinkedReferences);
     }
     EntityRefBuilder b = new EntityRefBuilder(slot: slot);
     Element typeElement = type.element;
     if (type is TypeParameterType) {
-      b.paramReference = findTypeParameterIndex(type, context);
+      int typeParameterIndex = findTypeParameterIndex(type, context);
+      if (typeParameterIndex != null) {
+        b.paramReference = typeParameterIndex;
+      } else {
+        // Out-of-scope type parameters only occur in circumstances where they
+        // are irrelevant (i.e. when a type parameter is unused).  So we can
+        // safely convert them to `dynamic`.
+        b.reference = serializeReferenceForType(DynamicTypeImpl.instance);
+      }
     } else if (type is FunctionType &&
         typeElement is FunctionElement &&
         typeElement.enclosingElement == null) {
       b.syntheticReturnType =
           serializeTypeRef(typeElement.returnType, typeElement);
       b.syntheticParams = typeElement.parameters
           .map((ParameterElement param) => serializeParam(param, context))
           .toList();
     } else {
       if (type is FunctionType &&
@@ skipping 599 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }