Give fields a model class in the service library.

runtime/bin/vmservice/observatory/test/string_escaping_test.dart

 // Copyright (c) 2014, 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 string_escaping_test;
 
 import 'dart:async';
 import 'dart:mirrors';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 31 matching lines @@
   for (int i = 0; i < 512; i++) longStringEven += "𝄞";
   longStringOdd = ".";
   for (int i = 0; i < 512; i++) longStringOdd += "𝄞";
 }
 
 var tests = [
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
     expectFullString(String varName, String varValueAsString) {
-      ServiceMap field = lib.variables.singleWhere((v) => v.name == varName);
-      Instance value = field['value'];
+      Field field = lib.variables.singleWhere((v) => v.name == varName);
+      Instance value = field.value;
       expect(value.valueAsString, equals(varValueAsString));
       expect(value.valueAsStringIsTruncated, isFalse);
     }
     expectTruncatedString(String varName, String varValueAsString) {
-      ServiceMap field = lib.variables.singleWhere((v) => v.name == varName);
-      Instance value = field['value'];
+      Field field = lib.variables.singleWhere((v) => v.name == varName);
+      Instance value = field.value;
       expect(varValueAsString, startsWith(value.valueAsString));
       expect(value.valueAsStringIsTruncated, isTrue);
     }
 
     script();  // Need to initialize variables in the testing isolate.
     expectFullString('ascii', ascii);
     expectFullString('latin1', latin1);
     expectFullString('unicode', unicode);
     expectFullString('hebrew', hebrew);
     expectFullString('singleQuotes', singleQuotes);
     expectFullString('doubleQuotes', doubleQuotes);
     expectFullString('newLines', newLines);
     expectFullString('tabs', tabs);
     expectFullString('suggrogatePairs', suggrogatePairs);
     expectFullString('nullInTheMiddle', nullInTheMiddle);  /// 01: ok
     expectTruncatedString('longStringEven', longStringEven);
     expectTruncatedString('longStringOdd', longStringOdd);
   }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);