Mustachio: cache property map for each type

[srawlins]

Currently the property maps are each a static function, rather than a getter, because they need to be type parameterized. In this case, however, they are built up every time a property needs to be accessed during mustache rendering. Here's one example, the property map for Package:

=> {
          ..._Renderer_LibraryContainer.propertyMap<CT_>(),
          ..._Renderer_Nameable.propertyMap<CT_>(),
          ..._Renderer_Locatable.propertyMap<CT_>(),
          ..._Renderer_Canonicalization.propertyMap<CT_>(),
          ..._Renderer_Warnable.propertyMap<CT_>(),
          'allLibraries': Property(...),
          ...

Package extends LibraryContainer, so it's property map is spread in first. Package then mixes in Nameable, Locatable, Canonicalization, and Warnable, so those are each spread in next. Finally, Package declares its own methods, like allLibraries, which are added last. Each property like allLibraries contains a few function literals. Overall, this Map is expensive to compute repeatedly.

By caching it on the context type, CT_, we generally build it very few times per renderer type. Generics don't play a huge role, so each renderer ultimately supports a small handful of types, typically one.

[coveralls]

Coverage remained the same at 91.741% when pulling fbabdc7 on srawlins:mustachio-s into f8d62e0 on dart-lang:master.

[jcollins-g]

LGTM, optional minor change only