update setAnalysisRoots to accept package spec file

pkg/analysis_server/tool/spec/spec_input.html

Index: pkg/analysis_server/tool/spec/spec_input.html
diff --git a/pkg/analysis_server/tool/spec/spec_input.html b/pkg/analysis_server/tool/spec/spec_input.html
index c26da51d1488261f377f832b21a5adeffc728a1b..fa4664f201fb06ee927f376824c982abc69f8719 100644
--- a/pkg/analysis_server/tool/spec/spec_input.html
+++ b/pkg/analysis_server/tool/spec/spec_input.html
@@ -615,17 +615,28 @@
               <value><ref>FilePath</ref></value>
             </map>
             <p>
-              A mapping from source directories to target directories
+              A mapping from source directories to package roots
               that should override the normal package: URI resolution
-              mechanism.  The analyzer will behave as though each
+              mechanism.
+            </p>
+            <p>
+              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 the target directory.  The
-              effect is the same as specifying the target directory as
+              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.
             </p>
             <p>
+              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.
+            </p>
+            <p>
               Files in any directories that are not overridden by this
               mapping have their package: URI's resolved using the
               normal pubspec.yaml mechanism.  If this field is absent,