Adds the Module transformers

[rbuckton]

The Module transformer is designed to transform most Module-specific syntax into compatible down-level JavaScript, including:

• Top-level Import Declarations
• Top-level Export Declarations
• Default exports and export assignments
• Exported functions and classes

The Module transformers include support for the following module kinds:

• CommonJS
• AMD
• UMD
• SystemJS
• ES6 (which is effectively a no-op).

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.

[sandersn]

If it's undefined, will it be skipped in the emit or emitted as undefined?

[rbuckton]

flattenNodes removes undefined nodes.

[sandersn]

without moduleName, this just generates require(), right? What will the larger generated statement be in that case, and is it meaningful?

[sandersn]

why does visitExportDeclaration save the result of getGeneratedNameForNode but visitImportDeclaration and visitImportEqualsDeclaration doesn't?

[rbuckton]

• visitExportDeclaration must always call getGeneratedNameForNode.
• visitImportDeclaration calls it conditionally.
• visitImportEqualsDeclaration doesn't call it at all.