Validate that root paths are absolute and normalized.

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

Index: pkg/analysis_server/lib/plugin/protocol/protocol.dart
diff --git a/pkg/analysis_server/lib/plugin/protocol/protocol.dart b/pkg/analysis_server/lib/plugin/protocol/protocol.dart
index 5617efc873f260fdab4d0c44d70b4b6f96af1df8..9daa9f05583ecf684d85ab7c79e9439e69c606f0 100644
--- a/pkg/analysis_server/lib/plugin/protocol/protocol.dart
+++ b/pkg/analysis_server/lib/plugin/protocol/protocol.dart
@@ -441,6 +441,15 @@ class Response {
                 "Invalid execution context: $contextId"));
 
   /**
+   * Initialize a newly created instance to represent the
+   * INVALID_FILE_PATH_FORMAT error condition.
+   */
+  Response.invalidFilePathFormat(Request request, path)
+      : this(request.id,
+            error: new RequestError(RequestErrorCode.INVALID_FILE_PATH_FORMAT,
+                'Invalid file path format: $path'));
+
+  /**
    * Initialize a newly created instance to represent an error condition caused
    * by a [request] that had invalid parameter.  [path] is the path to the
    * invalid parameter, in Javascript notation (e.g. "foo.bar" means that the