Make "Unbound type parameter" bugs easier to investigate.

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 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/member.dart';
@@ skipping 277 matching lines @@
     linkedUnit.types = deferredLinkedTypes
         .map((_SerializeTypeRef closure) => closure())
         .toList();
   }
 
   /**
    * Compute the appropriate De Bruijn index to represent the given type
    * parameter [type].
    */
   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');
+    throw new StateError(
+        'Unbound type parameter $type (${originalContext?.location})');
   }
 
   /**
    * 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 980 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }