[BUGFIX lts] Avoid excessively calling Glimmer AST transforms.

[rwjblue]

During the Ember 2.15 cycle glimmer-vm was updated and contained a new format for AST plugins. This required that all "legacy" plugins be wrapped (to avoid breakage of this intimate API). When the wrapping code was implemented two distinct bugs were introduced that would have caused custom AST transforms to be called many more times than in Ember < 2.15.

Bug 1: Accruing AST Transforms via re-registration

ember-cli-htmlbars allows addons to register AST plugins via the normal ember-cli preprocessor registry system. When this support was added it was not possible to provide plugins during a specific compilation, and therefore all plugins are registered globally (even by current ember-cli-htmlbars versions).

Due to the mechanism that ember-cli uses to build up the list of addons and dependencies for use at build time ember-cli-htmlbars ends up requiring the template compiler many times and subsequently registering any AST transform plugins that are present using the exported registerPlugin API. Since this tracks plugins in module state it is possible to have registerPlugin called many times with the same plugin. ember-cli-htmlbars attempts to mitigate the polution of plugins by forcing the require.cache of the template compiler module to be flushed. Unfortuneately, there are circumstances where this cache invalidation doesn't work and we therefore grow the number of registered AST plugins very quickly (once for each nested addon / engine / app that is in the project).

During the 2.15 glimmer-vm upgrade the previous deduping logic was removed (due to the fact that "legacy" AST transforms were always generating unique plugin instances). When combined with situations where ember-cli-htmlbars was not properly clearing the require cache the resulting build times of complex applications using AST plugins grew > double.

Bug 2: Legacy AST Transform Compat Bug

In order to avoid breakage in addons leveraging the intimate AST plugin APIs a wrapper layer was added. Unfortunately, the wrapper layer assumed that there would only be a single Program node. However, in glimmers AST every BlockStatement has a Program node (and may have an inverse with another Program node). This mistake meant that the legacy "wrapped" AST transforms would be instantiated and ran many times per-template which generates quite a bit of wasted work.

Fixes Included

• Bring back the deduplication code ensuring that a given AST plugin is only registered once.
• Fix the legacy AST transform wrapper code to only instantiate and invoke the legacy AST transform once for the outermost Program node.