Null-assert the value given to a Completer expecting a non-null value (#49053)

The code with out the null assertion is legal as per the type signature,
but will throw a runtime exception if the nullable value is null. To
make this exception more explicit, the value must be null-checked before
completing the completer with the value.

The analyzer will soon enforce such checks. See dart-lang/sdk#53253.

This PR is behaviorally a no-op.

Fixes flutter/flutter#136775


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Author: srawlins

lib/web_ui/test/engine/clipboard_test.dart

@@ -58,7 +58,7 @@ Future<void> testMain() async {
       const MethodCodec codec = JSONMethodCodec();
       final Completer<ByteData> completer = Completer<ByteData>();
       void callback(ByteData? data) {
-        completer.complete(data);
+        completer.complete(data!);
       }
 
       clipboardMessageHandler.setDataMethodCall(