Migration: Fix three broken FixBuilder unit tests.

These tests were all broken due to incorrect nullabilities in a
generic class method.  The brokenness was masked by issue #38900,
which was fixed in 1b044ee.

Change-Id: Ib77c437ae147afd10cc46e0685db25ce844bd99b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/121960
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441

pkg/nnbd_migration/test/fix_builder_test.dart

@@ -365,14 +365,11 @@ _f(_C<int, String> c) => c['foo'] = 1;
     visitAssignmentTarget(findNode.index('c['), null, 'int');
   }
 
-  @FailingTest(reason: r'''
-This test was broken because of considering  type parameter type nullabilities.
-''')
   test_assignmentTarget_indexExpression_substituted_check_rhs() async {
     await analyze('''
 class _C<T, U> {
   T operator[](U u) => throw 'foo';
-  void operator[]=(U/*?*/ u, T t) {}
+  void operator[]=(U/*!*/ u, T t) {}
 }
 _f(_C<int, String/*!*/> c, String/*?*/ s) => c[s] = 1;
 ''');
@@ -824,16 +821,10 @@ _f(_C<int, String> c) => c + 'foo';
     visitSubexpression(findNode.binary('c +'), 'int');
   }
 
-  @FailingTest(reason: r'''
-The code is resolved without NNBD, so the type of `T operator+(U u)` is
-`T* Function(U*)`. When we replace `T` with `int!`, and `U` with `String!` we
-get `int* Function(String*)`. Previously we substituted without considering
-type parameter type nullabilities.
-''')
   test_binaryExpression_userDefinable_substituted_check_rhs() async {
     await analyze('''
 class _C<T, U> {
-  T operator+(U u) => throw 'foo';
+  T operator+(U/*!*/ u) => throw 'foo';
 }
 _f(_C<int, String/*!*/> c, String/*?*/ s) => c + s;
 ''');
@@ -1028,13 +1019,10 @@ _f(_C<int, String> c) => c['foo'];
     visitSubexpression(findNode.index('c['), 'int');
   }
 
-  @FailingTest(reason: r'''
-This test was broken because of considering  type parameter type nullabilities.
-''')
   test_indexExpression_substituted_check_rhs() async {
     await analyze('''
 class _C<T, U> {
-  T operator[](U u) => throw 'foo';
+  T operator[](U/*!*/ u) => throw 'foo';
 }
 _f(_C<int, String/*!*/> c, String/*?*/ s) => c[s];
 ''');