Support returning Future<ExtensionType>

[xvrh]

Fixes #801

Have the generator correctly generate dummy return value for Future<ExtensionType>

Before, the generators would return: _i2.Future<_i3.Ext>.value((0)) (which won't compile). After, it will generate a cast from the representation dummy value: _i2.Future<_i3.Ext>.value((0 as _i3.Ext))

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