Do not mark type variables as needing a box inside try; they are immutable.

pkg/compiler/lib/src/closure.dart

 // Copyright (c) 2012, 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 closureToClassMapper;
 
 import 'constants/expressions.dart';
 import 'dart2jslib.dart';
 import 'dart_types.dart';
 import 'elements/elements.dart';
@@ skipping 633 matching lines @@
              variable.executableContext == executableContext;
     }
 
     if (insideClosure && !inCurrentContext(variable)) {
       closureData.addFreeVariable(variable);
     } else if (inTryStatement) {
       // Don't mark the this-element or a self-reference. This would complicate
       // things in the builder.
       // Note that nested (named) functions are immutable.
       if (variable != closureData.thisLocal &&
-          variable != closureData.closureElement) {
+          variable != closureData.closureElement &&
+          variable is! TypeVariableLocal) {
         closureData.variablesUsedInTryOrGenerator.add(variable);
       }
     } else if (variable is LocalParameterElement &&
         variable.functionDeclaration.asyncMarker == AsyncMarker.SYNC_STAR) {
       // Parameters in a sync* function are shared between each Iterator created
       // by the Iterable returned by the function, therefore they must be boxed.
       closureData.variablesUsedInTryOrGenerator.add(variable);
     }
   }
 
@@ skipping 446 matching lines @@
 
   String get name => typeVariable.name;
 
   int get hashCode => typeVariable.hashCode;
 
   bool operator ==(other) {
     if (other is! TypeVariableLocal) return false;
     return typeVariable == other.typeVariable;
   }
 }