Driver failure mode fixes (and test cleanup).

pkg/analyzer_cli/lib/src/options.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.
 
 library analyzer_cli.src.options;
 
 import 'dart:io';
 
 import 'package:analyzer_cli/src/driver.dart';
 import 'package:args/args.dart';
@@ skipping 128 matching lines @@
         if (sdkDir != null) {
           options.dartSdkPath = sdkDir.path;
         }
       }
 
       var sdkPath = options.dartSdkPath;
 
       // Check that SDK is specified.
       if (sdkPath == null) {
         printAndFail('No Dart SDK found.');
+        return null; // Only reachable in testing.
       }
       // Check that SDK is existing directory.
       if (!(new Directory(sdkPath)).existsSync()) {
         printAndFail('Invalid Dart SDK path: $sdkPath');
+        return null; // Only reachable in testing.
       }
     }
 
     // Check package config.
     {
       if (options.packageRootPath != null &&
           options.packageConfigPath != null) {
         printAndFail("Cannot specify both '--package-root' and '--packages.");
+        return null; // Only reachable in testing.
       }
     }
 
     // OK.  Report deprecated options.
     if (options.enableNullAwareOperators) {
       errorSink.writeln(
           "Info: Option '--enable-null-aware-operators' is no longer needed. "
           "Null aware operators are supported by default.");
     }
 
@@ skipping 313 matching lines @@
 
   int _getNextFlagIndex(args, i) {
     for (; i < args.length; ++i) {
       if (args[i].startsWith('--')) {
         return i;
       }
     }
     return i;
   }
 }