Add support for serializing 'target[index]' expressions.

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

Index: pkg/analyzer/lib/src/summary/summarize_const_expr.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
index 1671448a987071f434306abd72cd837cd77d904b..648b2dfd55e8bf25657496d6ecf76b3628d0bf10 100644
--- a/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_const_expr.dart
@@ -241,6 +241,10 @@ abstract class AbstractConstExprSerializer {
       _serializePropertyAccess(expr);
     } else if (expr is ParenthesizedExpression) {
       _serialize(expr.expression);
+    } else if (expr is IndexExpression) {
+      _serialize(expr.target);
+      _serialize(expr.index);
+      operations.add(UnlinkedConstOperation.extractIndex);
     } else {
       throw new StateError('Unknown expression type: $expr');
     }