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

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

Index: pkg/analyzer/test/src/context/builder_test.dart
diff --git a/pkg/analyzer/test/src/context/builder_test.dart b/pkg/analyzer/test/src/context/builder_test.dart
index 4569f8e0c1ac3c05cdef8047c8cf44b6273f4902..7cb396b2ea902ca7badf9335695af74a34088585 100644
--- a/pkg/analyzer/test/src/context/builder_test.dart
+++ b/pkg/analyzer/test/src/context/builder_test.dart
@@ -125,7 +125,7 @@ const Map<String, LibraryInfo> libraries = const {
 
     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,
         '''
@@ -304,6 +304,12 @@ bar:$barUri
     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);