Fix resolver for fields uses in constructors.

dart/frog/tests/leg_only/src/FieldInConstructorTest.dart

Index: dart/frog/tests/leg_only/src/FieldInConstructorTest.dart
===================================================================
--- dart/frog/tests/leg_only/src/FieldInConstructorTest.dart	(revision 0)
+++ dart/frog/tests/leg_only/src/FieldInConstructorTest.dart	(revision 0)
@@ -0,0 +1,21 @@
+// Copyright (c) 2011, 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.
+
+void expectEquals(var expected, var actual) {
+  if (expected != actual) {
+    print("Actual does not match expected");
+    throw actual;
+  }
+}
+
+class A {
+  var a;
+  A() {
+    a = 2;
+  }
+}
+
+main() {
+  expectEquals(2, new A().a);
+}