Add the analysis options path to the diagnostics page.

pkg/analyzer/lib/context/context_root.dart

Index: pkg/analyzer/lib/context/context_root.dart
diff --git a/pkg/analyzer/lib/context/context_root.dart b/pkg/analyzer/lib/context/context_root.dart
index cfc47ff8f99c2c0ebbadbf3ed75c5cbb49422994..0bd64e7b3e7aa9939f41fd83b2274b8c153f6d2c 100644
--- a/pkg/analyzer/lib/context/context_root.dart
+++ b/pkg/analyzer/lib/context/context_root.dart
@@ -24,6 +24,12 @@ class ContextRoot {
   final List<String> exclude;
 
   /**
+   * An informative value for the file path that the analysis options were read
+   * from. This value can be `null`.

[Brian Wilkerson, 2017/05/30 14:26:39]

Add a reason for 'null', such as "if there is no analysis options file or if the
location of the file has not yet been discovered".

[devoncarew, 2017/05/30 20:23:32]

Done.

+   */
+  String optionsFilePath;
+
+  /**
    * Initialize a newly created context root.
    */
   ContextRoot(this.root, this.exclude);