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

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 49c0eaec0eba6669f7b6559485ce84fbb58ad719..3beedab0e68ae4ffebca7e86d30ea4ce1c398e3d 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -651,7 +651,8 @@ class ClosureTranslator extends Visitor {
       // 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 &&