Migration: fix a corner case of post-dominator analysis.

When the variable on the LHS of an assignment expression also appears
in its RHS, we want the RHS to contribute to generation of hard edges,
because the RHS is evaluated prior to the assignment.  To achieve this
we need to remove the LHS variable from post-dominator scopes after
visiting the RHS.

Change-Id: Ifef9d69356e78e43f77ef456101b335ba3238312
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116059
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -273,7 +273,6 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       // TODO(paulberry)
       _unimplemented(node, 'Assignment with operator ${node.operator.lexeme}');
     }
-    _postDominatedLocals.removeReferenceFromAllScopes(node.leftHandSide);
     var expressionType = _handleAssignment(node.rightHandSide,
         destinationExpression: node.leftHandSide);
     var conditionalNode = _conditionalNodes[node.leftHandSide];
@@ -1513,6 +1512,9 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
         source: sourceType,
         destination: destinationType,
         hard: _postDominatedLocals.isReferenceInScope(expression));
+    if (destinationExpression != null) {
+      _postDominatedLocals.removeReferenceFromAllScopes(destinationExpression);
+    }
     if (destinationLocalVariable != null) {
       _flowAnalysis.write(destinationLocalVariable);
     }

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -3066,6 +3066,18 @@ void test(bool b1, bool b2, bool b3, bool _b) {
     assertEdge(decoratedTypeAnnotation('bool b3').node, never, hard: true);
   }
 
+  test_postDominators_assignment_with_same_var_on_lhs_and_in_rhs() async {
+    await analyze('''
+void f(int i) {
+  i = g(i);
+}
+int g(int j) => 0;
+''');
+    assertEdge(decoratedTypeAnnotation('int i').node,
+        decoratedTypeAnnotation('int j').node,
+        hard: true);
+  }
+
   test_postDominators_break() async {
     await analyze('''
 class C {