Add a priority to source changes for assists and fixes

pkg/analyzer_plugin/tool/spec/plugin_spec.html

Index: pkg/analyzer_plugin/tool/spec/plugin_spec.html
diff --git a/pkg/analyzer_plugin/tool/spec/plugin_spec.html b/pkg/analyzer_plugin/tool/spec/plugin_spec.html
index db51a25353ff0e0b54668ec8e5c722b0a5d4c24d..461ff9adf4bb64750c8f56a8cef27c3bd499db8b 100644
--- a/pkg/analyzer_plugin/tool/spec/plugin_spec.html
+++ b/pkg/analyzer_plugin/tool/spec/plugin_spec.html
@@ -597,7 +597,7 @@
     <result>
       <field name="assists">
         <list>
-          <ref>SourceChange</ref>
+          <ref>PrioritizedSourceChange</ref>
         </list>
         <p>
           The assists that are available at the given location.
@@ -891,7 +891,7 @@
         </p>
       </field>
       <field name="fixes">
-        <list><ref>SourceChange</ref></list>
+        <list><ref>PrioritizedSourceChange</ref></list>
         <p>
           The fixes associated with the error.
         </p>
@@ -1923,6 +1923,26 @@
       </field>
     </object>
   </type>
+  <type name="PrioritizedSourceChange">
+    <p>
+      A source change that has a priority associated with it.
+    </p>
+    <object>
+      <field name="priority">
+        <ref>int</ref>
+        <p>
+          The priority of the change. The value is expected to be non-negative,
+          and zero (0) is the lowest priority.
+        </p>
+      </field>
+      <field name="change">
+        <ref>SourceChange</ref>
+        <p>
+          The change with which the relevance is associated.
+        </p>
+      </field>
+    </object>
+  </type>
   <type name="RefactoringFeedback">
     <p>
       An abstract superclass of all refactoring feedbacks.