bug when sharing deferred constants

[sigmundch]

The deferred load splitting has issues when constants are shared among deferred chunks.

The following program illustrates the problem:

main.dart

import 'a.dart';
import 'b.dart';

main() async {
  (await doA()).method();
  await doB(); // comment out and the program works.
}

shared.dart

import 'c.dart';
const constant = const C(); // using `var` eludes the problem.

a.dart

import 'shared.dart' deferred as d;

doA() async {
  await d.loadLibrary();
  return d.constant;
}

b.dart

import 'shared.dart' deferred as d;

doB() async {
  await d.loadLibrary();
  return d.constant;
}

c.dart

class C {
  const C();
  method() => print("1");
}

This program fails with an exception when trying to call method on the first line of main. If you comment out the call to doB (hence there is no sharing), then the program runs fine.