Migration: substitute properly when handling compound assignments.

Change-Id: Ib8f26959688df3b2fc6aa2db66331ef0ec357794
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120500
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1643,8 +1643,9 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
               destination: _notNullType,
               hard: _postDominatedLocals
                   .isReferenceInScope(destinationExpression));
-          DecoratedType compoundOperatorType =
-              getOrComputeElementType(compoundOperatorMethod);
+          DecoratedType compoundOperatorType = getOrComputeElementType(
+              compoundOperatorMethod,
+              targetType: destinationType);
           assert(compoundOperatorType.positionalParameters.length > 0);
           _checkAssignment(edgeOrigin,
               source: sourceType,

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -641,7 +641,6 @@ C f(C y, C z) => (y += z);
     assertNullCheck(checkExpression('(y += z)'), fReturnEdge);
   }
 
-  @failingTest
   test_assignmentExpression_compound_withSubstitution() async {
     // Failing due to a side-cast from incorrectly instantiating the operator.
     var code = '''