Adopt dynamic-imports for node module

[jrieken]

With TypeScript 2.4 we can now use dynamic imports to improve startup performance. The idea is to lazy load certain modules that aren't (always) needed for starting up. Generally, lazy loading requires some amd-bundle-gymnastics but with node-modules everything is easy. The motivation is that 20% of code loading (from cached data) is spend loading node-modules.

To see how long it takes to load a certain module do the following:

• start code with the --performance flag
• in code hit F1 and show startup performance
• below the table with startup times are different sections outlining times to load and execute all modules we have. Look at everything taking 10ms or longer

In short, the idea here to explore using dynamic imports for lazying. Already today this can be done with late require-calls but you lose typing inference with that. Dynamic imports give you both, late and lazy loading and type safety/inference.

Sample times when not having cached data (very first startup, ~200 of 1500ms)

Sample times when having cached data (also node modules are cached, ~100 of 500ms)

[jrieken]

cc @Tyriar for evaluating what it means to load xterm only when needed.

[Tyriar]

@jrieken 6d71f77 removes it completely from startup, just need to work out how to pull in types without requiring the code, while still keeping them inside xterm.d.ts.

[jrieken]

Great. For having types without loading you can use some on-thin-ice-trick which is keeping the import xyzzy from 'xterm'-statement globally but making sure you reference nothing that exists at runtime. So, if you only reference interfaces and purely-declared-things there will be no global dependency in the emitted JS code. A proper variant of that might be have two d.ts-files, one for runtime types and one for interfaces et al.

[Tyriar]

Actually in the process of doing that exact thing microsoft/TypeScript#15902, getting a weird error though:

A proper variant of that might be have two d.ts-files

Soon the types will be handles entirely by the module, it would be a shame to have to keep them in Microsoft/vscode as well when that happens.

[Tyriar]

Opened microsoft/TypeScript#17200

[jrieken]

Closing this as most work is done except for xterm which is tracked in #31216