Adds the TypeScript transformer

[rbuckton]

The TypeScript transformer is designed to remove or transform most TypeScript-specific syntax.

This transformer elides TypeScript-specific nodes such as:

• Interfaces
• Type Aliases
• Type Annotations
• Ambient Declarations
• TypeScript-specific modifiers (e.g. abstract, public, private, etc.)
• implements heritage clauses on classes
• Type assertions
• Type parameter lists
• Type argument lists

In addition, it transforms TypeScript-specific syntax into compatible down-level syntax, including:

• Decorators
• Async Functions
• Property Declarations
• Parameter property initializers
• Enums
• Namespaces
• Exported declarations inside of a Namespace.

Some TypeScript-specific syntax will remain, specifically import and export declarations that will be handled directly by a later module transformation.

Related Pull Requests:

• Adds the visitor API foundation for tree transformations. #6892 - Adds the visitor API foundation for tree transformations.
• Adds the Transform Flags concept for tree transformations #6983 - Adds the Transform Flags concept for tree transformations.
• Adds the transformFiles API for tree transformations #6986 - Adds the transformFiles API for tree transformations.
• Adds a simplified pretty printer for tree transformations #6987 - Adds a simplified pretty printer for tree transformations.
• Adds the TypeScript transformer #7191 - Adds the TypeScript transformer.
• Adds the JSX transformer #7192 - Adds the JSX transformer.
• Adds the ES7 transformer #7193 - Adds the ES7 transformer.
• Adds the ES6 transformer #7194 - Adds the ES6 transformer.
• Adds the Module transformers #7206 - Adds the Module transformer.

[DanielRosenwasser]

Review status: 0 of 12 files reviewed at latest revision, 9 unresolved discussions, some commit checks failed.

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src/compiler/transformers/destructuring.ts, line 23 [r1] (raw file):
"highlight" is vague, I can't tell if this is for a future correspondence between transformations or sourcemaps or something else

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src/compiler/transformers/destructuring.ts, line 28 [r1] (raw file):
This gets a little hard to read. Please use complete sentences.

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src/compiler/transformers/destructuring.ts, line 40 [r1] (raw file):
I don't see the definition of inlineExpressions.

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src/compiler/transformers/destructuring.ts, line 231 [r1] (raw file):
Cache length, use below

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src/compiler/transformers/destructuring.ts, line 267 [r1] (raw file):
You already have numElements above

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src/compiler/transformers/destructuring.ts, line 279 [r1] (raw file):
numElements

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src/compiler/transformers/destructuring.ts, line 300 [r1] (raw file):
These are getting somewhat difficult to reason about. The variable is named object, but that's vague enough that I though this was an object literal or binding pattern for whatever reason.

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src/compiler/sourcemap.ts, line 70 [r1] (raw file):
I don't know what this is, leave a hefty comment on what it is and why I need it.

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src/compiler/transformer.ts, line 29 [r1] (raw file):
Should we conditionally transform modules? Or is it better to unconditionally transform just in case?

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Comments from the review on Reviewable.io

[DanielRosenwasser]

Review status: 0 of 12 files reviewed at latest revision, 10 unresolved discussions, some commit checks failed.

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src/compiler/transformers/ts.ts, line 210 [r4] (raw file):
Just write "they will be emitted as part of transformClassDeclaration"

If these constructs are taken care of by transformClassDeclaration, shouldn't these branches be unreachable?

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Comments from the review on Reviewable.io