--dump-info: only computeType if field used

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

Index: sdk/lib/_internal/compiler/implementation/dump_info.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dump_info.dart b/sdk/lib/_internal/compiler/implementation/dump_info.dart
index 93c3425da219cb7c19e6f9aa96df1b20468446fb..159b07ebc4141a20994379135a41276cc6a0800c 100644
--- a/sdk/lib/_internal/compiler/implementation/dump_info.dart
+++ b/sdk/lib/_internal/compiler/implementation/dump_info.dart
@@ -272,14 +272,14 @@ class InfoDumpVisitor extends ElementVisitor<InfoNode> {
 
   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(