ContextBuilder should to throw when a root folder does not exist.

pkg/analyzer/test/src/context/builder_test.dart

 // Copyright (c) 2016, 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.test.src.context.context_builder_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/command_line/arguments.dart';
 import 'package:analyzer/src/context/builder.dart';
@@ skipping 107 matching lines @@
     ArgResults argResults = argParser.parse(['--$enableStrictCallChecksFlag']);
     var builder = new ContextBuilder(resourceProvider, sdkManager, contentCache,
         options: createContextBuilderOptions(argResults));
 
     AnalysisOptionsImpl expected = new AnalysisOptionsImpl();
     expected.enableSuperMixins = true;
     expected.enableStrictCallChecks = true;
 
     String path = resourceProvider.convertPath('/some/directory/path');
     String filePath =
-    pathContext.join(path, AnalysisEngine.ANALYSIS_OPTIONS_YAML_FILE);
+        pathContext.join(path, AnalysisEngine.ANALYSIS_OPTIONS_YAML_FILE);
     resourceProvider.newFile(
         filePath,
         '''
 analyzer:
   language:
     enableSuperMixins : true
     enableStrictCallChecks : false
 ''');
 
     AnalysisOptions options = builder.getAnalysisOptions(path);
@@ skipping 158 matching lines @@
     expect(map['bar'], barUri);
   }
 
   void test_createPackageMap_none() {
     String rootPath = resourceProvider.convertPath('/root');
     resourceProvider.newFolder(rootPath);
     Packages packages = builder.createPackageMap(rootPath);
     expect(packages, same(Packages.noPackages));
   }
 
+  void test_createPackageMap_rootDoesNotExist() {
+    String rootPath = resourceProvider.convertPath('/root');
+    Packages packages = builder.createPackageMap(rootPath);
+    expect(packages, same(Packages.noPackages));
+  }
+
   void test_createSourceFactory_bazelWorkspace_fileProvider() {
     String _p(String path) => resourceProvider.convertPath(path);
 
     String projectPath = _p('/workspace/my/module');
     resourceProvider.newFile(_p('/workspace/WORKSPACE'), '');
     resourceProvider.newFolder(_p('/workspace/bazel-bin'));
     resourceProvider.newFolder(_p('/workspace/bazel-genfiles'));
     resourceProvider.newFolder(projectPath);
 
     AnalysisOptionsImpl options = new AnalysisOptionsImpl();
@@ skipping 434 matching lines @@
     expect(locator.embedderYamls, hasLength(0));
   }
 
   void test_valid() {
     EmbedderYamlLocator locator = new EmbedderYamlLocator({
       'fox': <Folder>[pathTranslator.getResource(foxLib)]
     });
     expect(locator.embedderYamls, hasLength(1));
   }
 }