Provide reasons for expect failures in service test helper.

tests/standalone/vmservice/test_helper.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.
 
 library vmservice_test_helper;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as JSON;
 import 'dart:utf' as UTF;
@@ skipping 15 matching lines @@
             .fold(new BytesBuilder(), (b, d) => b..add(d))
             .then((builder) {
               _requestCompleted(builder.takeBytes(), response);
             });
       }).catchError((error) {
         onRequestFailed(error);
       });
   }
 
   void _requestCompleted(List<int> data, HttpClientResponse response) {
-    Expect.equals(200, response.statusCode);
+    Expect.equals(200, response.statusCode, 'Invalid HTTP Status Code');
     var replyAsString;
     try {
       replyAsString = UTF.decodeUtf8(data, 0, null, null);
     } catch (e) {
       onRequestFailed(e);
       return;
     }
     var reply;
     try {
       reply = JSON.parse(replyAsString);
     } catch (e) {
       onRequestFailed(e);
       return;
     }
     if (reply is! Map) {
-        onRequestFailed('Reply was not a map: $reply');
-        return;
+      onRequestFailed('Reply was not a map: $reply');
+      return;
     }
     if (reply['type'] == null) {
-        onRequestFailed('Reply does not contain a type key: $reply');
-        return;
+      onRequestFailed('Reply does not contain a type key: $reply');
+      return;
     }
     try {
-        onRequestCompleted(reply);
-    } catch(e) {
-        onRequestFailed('Test callback failed: $e');
+      onRequestCompleted(reply);
+    } catch (e) {
+      onRequestFailed('Test callback failed: $e');
     }
   }
 
   void onRequestFailed(dynamic error) {
     Expect.fail('Failed to make request: $error');
   }
 
   void onRequestCompleted(Map response);
 }
 
@@ skipping 47 matching lines @@
   void requestExit() {
     process.stdin.add([32]);
   }
 }
 
 class IsolateListTester {
   final Map isolateList;
 
   IsolateListTester(this.isolateList) {
     // The reply is an IsolateList.
-    Expect.equals('IsolateList', isolateList['type']);
+    Expect.equals('IsolateList', isolateList['type'], 'Not an IsolateList.');
   }
 
   void checkIsolateCount(int n) {
-    Expect.equals(n, isolateList['members'].length);
+    Expect.equals(n, isolateList['members'].length, 'Isolate count not $n');
   }
 
   void checkIsolateIdExists(int id) {
     var exists = false;
     isolateList['members'].forEach((isolate) {
       if (isolate['id'] == id) {
         exists = true;
       }
     });
-    Expect.isTrue(exists);
+    Expect.isTrue(exists, 'No isolate with id: $id');
   }
 
   void checkIsolateNameContains(String name) {
     var exists = false;
     isolateList['members'].forEach((isolate) {
       if (isolate['name'].contains(name)) {
         exists = true;
       }
     });
-    Expect.isTrue(exists);
+    Expect.isTrue(exists, 'No isolate with name: $name');
   }
 
   void checkIsolateNamePrefix(int id, String name) {
     var exists = false;
     isolateList['members'].forEach((isolate) {
       if (isolate['id'] == id) {
         exists = true;
-        Expect.isTrue(isolate['name'].startsWith(name));
+        Expect.isTrue(isolate['name'].startsWith(name),
+                      'Isolate $id does not have name prefix: $name');
       }
     });
-    Expect.isTrue(exists);
+    Expect.isTrue(exists, 'No isolate with id: $id');
   }
 }