Make findElementReferences' "id" result optional.

editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java

Index: editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java
diff --git a/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java b/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java
index 434566a5771d4eee0e3a2c61c64b2290a75a91d3..6f1ffe554730313a3be61a442424a333d1bfe893 100644
--- a/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java
+++ b/editor/tools/plugins/com.google.dart.server/src/com/google/dart/server/internal/remote/processor/SearchIdProcessor.java
@@ -14,6 +14,7 @@
 package com.google.dart.server.internal.remote.processor;
 
 import com.google.dart.server.SearchIdConsumer;
+import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
 
 /**
@@ -31,6 +32,7 @@ public class SearchIdProcessor extends ResultProcessor {
   }
 
   public void process(JsonObject resultObject) {
-    consumer.computedSearchId(resultObject.get("id").getAsString());
+    JsonElement jsonElement = resultObject.get("id");
+    consumer.computedSearchId(jsonElement != null ? jsonElement.getAsString() : null);
   }
 }