update setAnalysisRoots to accept package spec file

pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart

Index: pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart
diff --git a/pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart b/pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart
index d90344f953523ee0a65d53ee1379f9d4460ea6fd..eab48cdcaf37e0a48500a815033cf56c49779c17 100644
--- a/pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart
+++ b/pkg/analysis_server/lib/plugin/protocol/generated_protocol.dart
@@ -1663,12 +1663,19 @@ class AnalysisSetAnalysisRootsParams implements HasToJson {
   }
 
   /**
-   * A mapping from source directories to target directories that should
-   * override the normal package: URI resolution mechanism. The analyzer will
-   * behave as though each source directory in the map contains a special
-   * pubspec.yaml file which resolves any package: URI to the corresponding
-   * path within the target directory. The effect is the same as specifying the
-   * target directory as a "--package_root" parameter to the Dart VM when
+   * A mapping from source directories to package roots that should override
+   * the normal package: URI resolution mechanism.
+   *
+   * If a package root is a directory, then the analyzer will behave as though
+   * the associated source directory in the map contains a special pubspec.yaml
+   * file which resolves any package: URI to the corresponding path within that
+   * package root directory. The effect is the same as specifying the package
+   * root directory as a "--package_root" parameter to the Dart VM when
+   * executing any Dart file inside the source directory.
+   *
+   * If a package root is a file, then the analyzer will behave as though that
+   * file is a ".packages" file in the source directory. The effect is the same
+   * as specifying the file as a "--packages" parameter to the Dart VM when
    * executing any Dart file inside the source directory.
    *
    * Files in any directories that are not overridden by this mapping have
@@ -1679,12 +1686,19 @@ class AnalysisSetAnalysisRootsParams implements HasToJson {
   Map<String, String> get packageRoots => _packageRoots;
 
   /**
-   * A mapping from source directories to target directories that should
-   * override the normal package: URI resolution mechanism. The analyzer will
-   * behave as though each source directory in the map contains a special
-   * pubspec.yaml file which resolves any package: URI to the corresponding
-   * path within the target directory. The effect is the same as specifying the
-   * target directory as a "--package_root" parameter to the Dart VM when
+   * A mapping from source directories to package roots that should override
+   * the normal package: URI resolution mechanism.
+   *
+   * If a package root is a directory, then the analyzer will behave as though
+   * the associated source directory in the map contains a special pubspec.yaml
+   * file which resolves any package: URI to the corresponding path within that
+   * package root directory. The effect is the same as specifying the package
+   * root directory as a "--package_root" parameter to the Dart VM when
+   * executing any Dart file inside the source directory.
+   *
+   * If a package root is a file, then the analyzer will behave as though that
+   * file is a ".packages" file in the source directory. The effect is the same
+   * as specifying the file as a "--packages" parameter to the Dart VM when
    * executing any Dart file inside the source directory.
    *
    * Files in any directories that are not overridden by this mapping have