fix nnbd websocket

This fixes web_socket_typed_data_test and web_socket_test

Change-Id: I7200ba423dbe24857854ba600f9fb9b3d4757635
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/135620
Commit-Queue: Zichang Guo <zichangguo@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

pkg/dev_compiler/tool/dart2js_nnbd_sdk_error_golden.txt

@@ -12,8 +12,8 @@ ERROR|STATIC_TYPE_WARNING|UNDEFINED_OPERATOR|lib/_internal/js_runtime/lib/interc
 ERROR|STATIC_TYPE_WARNING|UNDEFINED_OPERATOR|lib/_internal/js_runtime/lib/interceptors.dart|1666|18|1|The operator '&' isn't defined for the type 'JSInt'.
 ERROR|STATIC_TYPE_WARNING|UNDEFINED_OPERATOR|lib/_internal/js_runtime/lib/interceptors.dart|1666|44|1|The operator '&' isn't defined for the type 'JSInt'.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|1476|39|5|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8383|60|5|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9310|54|5|The left operand can't be null, so the right operand is never executed.
+WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8384|60|5|The left operand can't be null, so the right operand is never executed.
+WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9311|54|5|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/developer/developer.dart|315|25|23|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/html/dart2js/html_dart2js.dart|4075|25|2|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/io/io.dart|9167|16|1|The left operand can't be null, so the right operand is never executed.

pkg/dev_compiler/tool/dartdevc_nnbd_sdk_error_golden.txt

@@ -19,8 +19,8 @@ ERROR|COMPILE_TIME_ERROR|BODY_MAY_COMPLETE_NORMALLY|lib/html/dart2js/html_dart2j
 ERROR|COMPILE_TIME_ERROR|BODY_MAY_COMPLETE_NORMALLY|lib/html/dart2js/html_dart2js.dart|2978|13|11|The body might complete normally, which would cause 'null' to be returned, but the return type is a potentially non-nullable type.
 ERROR|COMPILE_TIME_ERROR|BODY_MAY_COMPLETE_NORMALLY|lib/html/dart2js/html_dart2js.dart|41436|8|15|The body might complete normally, which would cause 'null' to be returned, but the return type is a potentially non-nullable type.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|1476|39|5|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8383|60|5|The left operand can't be null, so the right operand is never executed.
-WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9310|54|5|The left operand can't be null, so the right operand is never executed.
+WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|8384|60|5|The left operand can't be null, so the right operand is never executed.
+WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/_http/http.dart|9311|54|5|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/collection/collection.dart|1076|46|13|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/developer/developer.dart|332|25|23|The left operand can't be null, so the right operand is never executed.
 WARNING|STATIC_WARNING|DEAD_NULL_COALESCE|lib/html/dart2js/html_dart2js.dart|4075|25|2|The left operand can't be null, so the right operand is never executed.

sdk_nnbd/lib/_http/http_headers.dart

@@ -764,7 +764,8 @@ class _HeaderValue implements HeaderValue {
               index++;
               break;
             }
-            sb.write(s[index]);
+            char = s[index];
+            sb.write(char);
             index++;
           }
           return sb.toString();

sdk_nnbd/lib/_http/websocket_impl.dart

@@ -710,8 +710,10 @@ class _WebSocketOutgoingTransformer
       } else {
         throw new ArgumentError(message);
       }
-
-      _deflateHelper?.processOutgoingMessage(messageData);
+      var deflateHelper = _deflateHelper;
+      if (deflateHelper != null) {
+        messageData = deflateHelper.processOutgoingMessage(messageData);
+      }
       data = messageData;
     } else {
       opcode = _WebSocketOpcode.TEXT;