Fix test.py and tests to use an explicit --packages flag.

pkg/analyzer/tool/task_dependency_graph/generate.dart

Index: pkg/analyzer/tool/task_dependency_graph/generate.dart
diff --git a/pkg/analyzer/tool/task_dependency_graph/generate.dart b/pkg/analyzer/tool/task_dependency_graph/generate.dart
index 068790f5787d3e45e9c50f3d6ca8ed47e5a4978c..9761e90d0675c02a1593398b02afbb0bd2b0b05c 100644
--- a/pkg/analyzer/tool/task_dependency_graph/generate.dart
+++ b/pkg/analyzer/tool/task_dependency_graph/generate.dart
@@ -156,9 +156,11 @@ ${generateGraphData()}
     context = AnalysisEngine.instance.createAnalysisContext();
     ContextBuilder builder = new ContextBuilder(resourceProvider, null, null);
     if (Platform.packageRoot != null) {
-      builder.defaultPackagesDirectoryPath = Uri.parse(Platform.packageRoot).toFilePath();
+      builder.defaultPackagesDirectoryPath =
+        Uri.parse(Platform.packageRoot).toFilePath();
     } else if (Platform.packageConfig != null) {
-      builder.defaultPackageFilePath = Platform.packageConfig;
+      builder.defaultPackageFilePath =
+        Uri.parse(Platform.packageConfig).toFilePath();
     } else {
       // Let the context builder use the default algorithm for package
       // resolution.