add babel-plugin-transform-block-scoping

[erwinmombay]

A discussion in evanw/esbuild#478 has shown that using let and class in the same enclosing/block level has a significant performance impact on the Safari browser when there is a large number of instances of the block scoped variables to track (such as a large bundle with a large number of let/const/class declarations). To alleviate this problem we transform the let and class constructs into var when it is 'easily' possible (ex. we bail on the for loop block scoping to not complicate the transform).

this should only be turned on for esm/sxg in post closure transforms

We do not do this transform to const as we do a const to let transformer prior to this transform. As a follow up PR i will combine this with the let to const transformer

Co-authored-by: Justin Ridgewell jridgewell@google.com
Co-authored-by: erwin mombay erwinm@google.com

[amp-owners-bot]

Hey @jridgewell! These files were changed:

build-system/babel-plugins/babel-plugin-transform-block-scoping/index.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-class-declaration/transform-class/input.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-class-declaration/transform-class/options.json
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-class-declaration/transform-class/output.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/no-transform-const/input.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/no-transform-const/options.json
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/no-transform-const/output.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/transform-let/input.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/transform-let/options.json
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/fixtures/transform-variable-declaration/transform-let/output.js
build-system/babel-plugins/babel-plugin-transform-block-scoping/test/index.js

[kristoferbaxter]

Agree with @jridgewell feedback that we'll need to turn on mangle here now (https://github.com/ampproject/amphtml/blob/master/build-system/compile/post-closure-babel.js#L33).

[kristoferbaxter]

Mangle will be needed to rename the variable references.

[erwinmombay]

size before this change:
Computing bundle size for version 2010212121000 at commit 6418ea9.
brotli bundle sizes are:
dist/v0.mjs: 59.49KB

size after this change:
Computing bundle size for version 2010220007000 at commit 9799f73.
brotli bundle sizes are:
dist/v0.mjs: 59.48KB