RemoveNonNullAssertion fix removes necessary whitespace afterwards

[oprypin]

Example code before:

SubFoo f(Foo foo) {
  return foo! as SubFoo;
}

Example code after the fixer:

SubFoo f(Foo foo) {
  return fooas SubFoo;
}

Relevant imlpementation

sdk/pkg/analysis_server/lib/src/services/correction/dart/remove_non_null_assertion.dart

Lines 34 to 36 in 86d1261

	await builder.addDartFileEdit(file, (builder) {
	builder.addDeletion(range.startStart(bangToken, bangToken.next!));
	});