Fix simple type inferrer regression by using typeOf instead of returnTypeOf for final fields.

tests/compiler/dart2js/simple_inferrer_final_field_test.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.
+
+import 'compiler_helper.dart';
+import 'parser_helper.dart';
+
+const String TEST = """
+
+class A {
+  final intField;
+  final giveUpField;
+  A() : intField = 42, giveUpField = 'foo';
+  A.bar() : intField = 54, giveUpField = 42;
+}
+
+main() {
+  new A();
+  new A.bar();
+}
+""";
+
+void main() {
+  Uri uri = new Uri.fromComponents(scheme: 'source');
+  var compiler = compilerFor(TEST, uri);
+  compiler.runCompiler(uri);
+  var typesInferrer = compiler.typesTask.typesInferrer;
+
+  checkFieldTypeInClass(String className, String fieldName, type) {
+    var cls = findElement(compiler, className);
+    var element = cls.lookupLocalMember(buildSourceString(fieldName));
+    Expect.equals(type, typesInferrer.typeOf[element]);
+  }
+
+  checkFieldTypeInClass('A', 'intField', compiler.intClass);
+  checkFieldTypeInClass('A', 'giveUpField', typesInferrer.giveUpType);
+}