[web] Split initialization of the _flutter namespace and loader object.

[ditman]

This makes it possible to more flexibly compose {{flutter_js}} and {{flutter_build_options}} in @eyebrowsoffire's bootstrapping improvements PR.

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[ditman]

There's several tests covering this gets initialized correctly; not sure if we have a way to test that this is initialized slightly differently though.

[eyebrowsoffire]

LGTM!

[ditman]

This is part of our test bootstrap here:

• engine/lib/web_ui/dev/test_platform.dart

  Lines 556 to 559 in eef5247

  	final String bootstrapScript = '''
  	<script src="/flutter_js/flutter.js"></script>
  	<script>
  	_flutter.buildConfig = {

Unfortunately, I cannot add any code that runs before the bootstrap; my idea was to

_flutter = {
  miscellaneous: 'value',
};

then run the rest of the test, but by the time the main method of my test is running, _flutter and _flutter.loader are already defined.

[ditman]

Another way is to have _flutter.miscellaneous = 'value' defined in the bootstrap before flutter.js, and see that nothing breaks, which I'm going to do right now.

[ditman]

By tweaking the test_platform.dart, actually all tests are testing this feature now. Removing missing test warning.

[ditman]

It seems my test to the platform isn't liked by some web tests? Wat?

[ditman]

Reproduced locally, and found JS error in strict mode where my: if (_flutter == null) was throwing an Uncaught ReferenceError: _flutter is not defined error...

So replaced the initialization to the exact same thing we use in flutter.js: if (!window._flutter)

That should do the trick.

[ditman]

Created an issue while investigating this flutter/flutter#144969