Introduce a get_dictionary_property function to reduce duplicated code in the dictionary conversion codegen.

This also explicitly disallows dictionary members without a default value, as
the code for those doesn't currently compile.

This is the second step of my planned rewrite of the dictionary initialization
that will remove the default values we currently use to initialize the
dictionary struct in the 'new' function.
auto merge of #2167 : Ms2ger/servo/get_dictionary_property, r=jdm

...the dictionary conversion codegen.

This also explicitly disallows dictionary members without a default value, as
the code for those doesn't currently compile.

This is the second step of my planned rewrite of the dictionary initialization
that will remove the default values we currently use to initialize the
dictionary struct in the 'new' function.