Add ElementInfo.id check fr null to debug flakiness on bots.

pkg/analyzer/lib/src/summary/index_unit.dart

Index: pkg/analyzer/lib/src/summary/index_unit.dart
diff --git a/pkg/analyzer/lib/src/summary/index_unit.dart b/pkg/analyzer/lib/src/summary/index_unit.dart
index c2c298e513ce0a40ce20dca4b8a390eb420ef460..ed3097dcdac801bf0b0f997e3cc0f8bfa958c33b 100644
--- a/pkg/analyzer/lib/src/summary/index_unit.dart
+++ b/pkg/analyzer/lib/src/summary/index_unit.dart
@@ -102,7 +102,7 @@ class PackageIndexAssembler {
         offset = 0;
       }
       IndexSyntheticElementKind kind = getIndexElementKind(element);
-      return new _ElementInfo(unitId, offset, kind);
+      return new _ElementInfo(element, unitId, offset, kind);
     });
   }
 
@@ -193,6 +193,11 @@ class _DefinedNameInfo {
  */
 class _ElementInfo {
   /**
+   * TODO(scheglov) remove after fixing build flakiness on bots
+   */
+  final Element element;
+
+  /**
    * The identifier of the [CompilationUnitElement] containing this element.
    */
   final int unitId;
@@ -213,7 +218,7 @@ class _ElementInfo {
    */
   int id;
 
-  _ElementInfo(this.unitId, this.offset, this.kind);
+  _ElementInfo(this.element, this.unitId, this.offset, this.kind);
 }
 
 /**
@@ -587,6 +592,10 @@ class _UnitIndexAssembler {
    */
   UnitIndexBuilder assemble() {
     relations.sort((a, b) {
+      if (a.elementInfo.id == null) {
+        throw new StateError('No id for ${a.elementInfo.element}, '
+            'location= ${a.elementInfo.element?.location}');
+      }
       return a.elementInfo.id - b.elementInfo.id;
     });
     definedNames.sort((a, b) {