Better reporting of ambiguous and "not found" commands in Obs debugger.

runtime/observatory/tests/service/command_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'dart:async';
 
 import 'package:observatory/cli.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 167 matching lines @@
       }));
   }));
 }
 
 void testCommandRunNotFound() {
   // Run a simple command.
   StringBuffer out = new StringBuffer();
   RootCommand cmd = new RootCommand([new TestCommand(out, 'alpha', [])]);
 
   cmd.runCommand('goose').catchError(expectAsync((e) {
-      expect(e, equals('No such command'));
+    expect(e.toString(), equals("No such command: 'goose'"));
   }));
 }
 
 void testCommandRunAmbiguous() {
   // Run a simple command.
   StringBuffer out = new StringBuffer();
   RootCommand cmd = new RootCommand([new TestCommand(out, 'alpha', []),
                                      new TestCommand(out, 'ankle', [])]);
 
   cmd.runCommand('a 55').catchError(expectAsync((e) {
-      expect(e, equals('Ambiguous command'));
+      expect(e.toString(),
+             equals("Command 'a 55' is ambiguous: [alpha, ankle]"));
       out.clear();
       cmd.runCommand('ankl 55').then(expectAsync((_) {
           expect(out.toString(), equals('executing ankle([55])\n'));
       }));
   }));
 }
 
 void testCommandRunAlias() {
   // Run a simple command.
   StringBuffer out = new StringBuffer();
   var aliasCmd = new TestCommand(out, 'alpha', []);
   aliasCmd.alias = 'a';
   RootCommand cmd = new RootCommand([aliasCmd,
                                      new TestCommand(out, 'ankle', [])]);
 
   cmd.runCommand('a 55').then(expectAsync((_) {
     expect(out.toString(), equals('executing alpha([55])\n'));
   }));
 }
 
 main() {
   test('command completion test suite', testCommandComplete);
   test('run a simple command', testCommandRunSimple);
   test('run a subcommand', testCommandRunSubcommand);
   test('run a command which is not found', testCommandRunNotFound);
   test('run a command which is ambiguous', testCommandRunAmbiguous);
   test('run a command using an alias', testCommandRunAlias);
 }