Infer type of extract index expressions.

pkg/analyzer/test/src/summary/resynthesize_ast_test.dart

Index: pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
index 408db12bcc9ce3c0e8dd32cd8cadd6737ff1a059..5e2bcd6adc92ffcb88ac8fff3ee9a06ca722bbae 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
@@ -209,6 +209,33 @@ class AstInferredTypeTest extends AbstractResynthesizeTest
     super.test_genericMethods_inferJSBuiltin();
   }
 
+  void test_infer_extractIndex_custom() {
+    var unit = checkFile('''
+class A {
+  String operator [](_) => null;
+}
+var a = new A();
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'String');
+  }
+
+  void test_infer_extractIndex_fromList() {
+    var unit = checkFile('''
+var a = <int>[1, 2, 3];
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'int');
+  }
+
+  void test_infer_extractIndex_fromMap() {
+    var unit = checkFile('''
+var a = <int, double>{};
+var b = a[0];
+  ''');
+    expect(unit.topLevelVariables[1].type.toString(), 'double');
+  }
+
   void test_infer_extractProperty_getter() {
     checkFile(r'''
 var a = 1.isEven;