--dump-info: only computeType if field used

sdk/lib/_internal/compiler/implementation/dump_info.dart

 // Copyright (c) 2013, 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 dump_info;
 
 import 'elements/elements.dart';
 import 'elements/visitor.dart';
 import 'dart:convert' show HtmlEscape;
 import 'dart2jslib.dart' show
@@ skipping 254 matching lines @@
     return element.alias == null
         ? null
         : new ElementInfoNode(
             type: element.alias.toString(),
             kind: "typedef",
             name: element.name);
   }
 
   InfoNode visitFieldElement(FieldElement element) {
     CodeBuffer emittedCode = compiler.backend.codeOf(element);
-    int size = 0;
-    DartType type = element.computeType(compiler);
     TypeMask inferredType = compiler.typesTask
         .getGuaranteedTypeOfElement(element);
     // If a field has an empty inferred type it is never used.
     if ((inferredType == null || inferredType.isEmpty) && emittedCode == null) {
       return null;
     }
+    int size = 0;
+    DartType type = element.computeType(compiler);
     List<InfoNode> contents = new List<InfoNode>();
     if (emittedCode != null) {
       contents.add(new CodeInfoNode(
           description: "Generated initializer",
           generatedCode: emittedCode.getText()));
       size = emittedCode.length;
     }
     if (inferredType != null) {
       contents.add(new InferredInfoNode(
           description: "type",
@@ skipping 215 matching lines @@
         var container = containers[i];
         container.addEventListener('click',
           toggler(container.nextElementSibling), false);
        container.nextElementSibling.hidden = true;
       };
     </script>
   </body>
 </html>""");
   }
 }