Change AssignmentExpression tests to assertAssignment().

Change-Id: If7cbb0584bfbf248ea79fe827626f4189d2f1c39
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/161041
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/generated/element_resolver.dart

@@ -1409,6 +1409,7 @@ class ElementResolver extends SimpleAstVisitor<void> {
           var getter = typeReference.getGetter(propertyName.name);
           if (getter != null) {
             propertyName.staticElement = getter;
+            _resolver.setWriteElement(node, getter);
             // The error will be reported in ErrorVerifier.
           } else {
             _errorReporter.reportErrorForNode(
@@ -1570,6 +1571,7 @@ class ElementResolver extends SimpleAstVisitor<void> {
         } else {
           var getter = result.getter;
           propertyName.staticElement = getter;
+          _resolver.setWriteElement(node, getter);
           // A more specific error will be reported in ErrorVerifier.
         }
       } else if (result.isNone) {