Saving scoped object outside scope in Dart - dart

Question

The following leaves s null after the file read exits:
String s;
new File('etc.stk').readAsString().then((String contents) {
s = contents;
});
// s is null here.
Is there a way to save (or clone) s, or am I compelled to use it only in the .then scope?
I have a few thousand lines of compiler/interpreter code that parses and runs the file contents, and would prefer not to have them all inside the new File scope.
EDIT
To provide more context, what I am trying to do is something like
new File('etc1.stk').readAsString()
.then((String script) {
syntaxTree1 = buildTree(script);
});
new File('etc2.stk').readAsString()
.then((String script) {
syntaxTree2 = buildTree(script);
});
and have access to both syntaxTree1 and syntaxTree2 in subsequent code. I will wrap my mind around the Dart Way if I can.

Answer 1

EDIT
(this code is tested)
import 'dart:async' as async;
import 'dart:io' as io;
void main(args) {
// approach1: inline
async.Future.wait([
new io.File('file1.txt').readAsString(),
new io.File('file2.txt').readAsString()
]).then((values) {
values.forEach(print);
});
// approach2: load files in another function
getFiles().then((values) {
values.forEach(print);
});
}
async.Future<List> getFiles() {
return async.Future.wait([
new io.File('file1.txt').readAsString(),
new io.File('file2.txt').readAsString()
]);
}
output:
file1
file2
file1
file2
EDIT END
hint: the code is not tested
// s is null here
is because this line is executed before
s = contents
This code
new File('etc.stk').readAsString()
returns a future that is enlisted in the event queue and is executed when the actual 'thread' of execution is finished.
If you had provided more code I would have a better context for a proposed solution.
What you could do is
String s;
new File('etc.stk').readAsString().then((String contents) {
s = contents;
}).then((_) {
// s is **NOT** null here.
});
or
//String s;
new File('etc.stk').readAsString().then((String contents) {
//s = contents;
someCallback(s)
});
// s is null here.
void someCallback(String s) {
// s is **NOT** null here
}
or
Future<String> myReadAsString() {
return new File('etc.stk').readAsString();
}
myReadAsString().then((s) {
// s is **NOT** null here
}
see also:
https://www.dartlang.org/slides/2013/06/dart-streams-are-the-future.pdf
async programming in dart
https://www.dartlang.org/docs/tutorials/#futures
https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:async
https://www.dartlang.org/articles/event-loop/
https://www.dartlang.org/articles/futures-and-error-handling/
and maybe
https://www.dartlang.org/articles/creating-streams/