Give fields a model class in the service library.

runtime/bin/vmservice/observatory/test/contexts_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 inbound_references_test;
 
 import 'dart:async';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
@@ skipping 34 matching lines @@
   cleanBlock = genCleanBlock();
   copyingBlock = genCopyingBlock();
   fullBlock = genFullBlock();
   fullBlockWithChain = genFullBlockWithChain();
 }
 
 var tests = [
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    ServiceMap field = lib.variables.singleWhere((v) => v.name == 'cleanBlock');
-    return field['value'].load().then((Instance block) {
+    Field field = lib.variables.singleWhere((v) => v.name == 'cleanBlock');
+    return field.value.load().then((Instance block) {
       expect(block.isClosure, isTrue);
       expect(block.closureCtxt.isContext, isTrue);
       expect(block.closureCtxt.length, equals(0));
       return block.closureCtxt.load().then((Context ctxt) {
         expect(ctxt.parentContext.isNull, isTrue);
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    ServiceMap field = lib.variables.singleWhere((v) => v.name == 'copyingBlock');
-    return field['value'].load().then((Instance block) {
+    Field field = lib.variables.singleWhere((v) => v.name == 'copyingBlock');
+    return field.value.load().then((Instance block) {
       expect(block.isClosure, isTrue);
       expect(block.closureCtxt.isContext, isTrue);
       expect(block.closureCtxt.length, equals(1));
       return block.closureCtxt.load().then((Context ctxt) {
         expect(ctxt.variables.single['value'].isString, isTrue);
         expect(ctxt.variables.single['value'].valueAsString, equals('I could be copied into the block'));
         expect(block.closureCtxt.parentContext.isNull, isTrue);
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    ServiceMap field = lib.variables.singleWhere((v) => v.name == 'fullBlock');
-    return field['value'].load().then((Instance block) {
+    Field field = lib.variables.singleWhere((v) => v.name == 'fullBlock');
+    return field.value.load().then((Instance block) {
       expect(block.isClosure, isTrue);
       expect(block.closureCtxt.isContext, isTrue);
       expect(block.closureCtxt.length, equals(1));
       return block.closureCtxt.load().then((ctxt) {
         expect(ctxt.variables.single['value'].isInt, isTrue);
         expect(ctxt.variables.single['value'].valueAsString, equals('43'));
         expect(ctxt.parentContext.isNull, isTrue);
       });
     });
   }),
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
-    ServiceMap field = lib.variables.singleWhere((v) => v.name == 'fullBlockWithChain');
-    return field['value'].load().then((Instance block) {
+    Field field = lib.variables.singleWhere((v) => v.name == 'fullBlockWithChain');
+    return field.value.load().then((Instance block) {
       expect(block.isClosure, isTrue);
       expect(block.closureCtxt.isContext, isTrue);
       expect(block.closureCtxt.length, equals(1));
       return block.closureCtxt.load().then((Context ctxt) {
         expect(ctxt.variables.single['value'].isInt, isTrue);
         expect(ctxt.variables.single['value'].valueAsString, equals('4201'));
         expect(ctxt.parentContext.isContext, isTrue);
         expect(ctxt.parentContext.length, equals(1));
         return ctxt.parentContext.load().then((Context outerCtxt) {
           expect(outerCtxt.variables.single['value'].isInt, isTrue);
           expect(outerCtxt.variables.single['value'].valueAsString, equals('421'));
           expect(outerCtxt.parentContext.isNull, isTrue);
         });
       });
     });
   }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);