Convert to ES Modules

[rachel-fenichel]

Closure compiler documentation:

• ES6 modules and Closure interop
• Migrating from goog.modules to ES6 modules

The specific steps that we need to take, pulled from the closure documentation:
Migrating Closure modules with named exports
For Closure modules that use named exports we recommend migrating files in place over 3 changes.

1. Edit the implementation file to be an ES module that calls goog.declareModuleId with the old Closure module name (i.e. the old argument to goog.module). This should allow other files to continue using goog.require on the Closure namespace. (Be sure to import Closure's goog.js file rather than use the global goog symbol!)
2. Edit all ES module files that are using goog.require on the module namespace to instead use ES import.
3. Remove the call to goog.declareModuleId in the migrated file once it is not being referenced by any goog.module or goog.provide files.

Reasons to migrate

• (As I understand it) the Typescript compiler cannot pick up and use the correct types for imports in the form of const {Foo} = goog.require('Blockly.Foo').
• Tsickle assumes that the input files are ES modules.
• Tsickle relies on correct parsing by the typescript compiler, and chokes when the compiler chokes.
• The closure compiler understands ES modules, so in theory this change can be done without involving the typescript compiler--meeting my goal of being able to compiler with the closure compiler up until we've fully debugged our typescript compilation flow.

[cpcallen]

Title tweaked: although they were introduced in ES6, they are more widely referred to simply as "ES modules" rather than "ES6 modules". (Various useful features like dynamic import() have also been added more recently.)

[cpcallen]

1. (Be sure to import Closure's goog.js file rather than use the global goog symbol!)

Have you verified that this is necessary?

[rachel-fenichel]

We may be able to skip directly to TypeScript, in which case we can close this entirely. @gonfunko is currently investigating this.