(Note the preload implementation is based off of old SystemJS which was designed to support browsers at the time... this is probably a good thing for compat, but full current browser support still needs to be verified!)

I don't want to land this until it is clear that this is a direction we want to encourage. I'm still doing perf tests on various real-world use cases. Until the major benefit can be seen I'll leave this up as a WIP.

I can't wait to use this feature!
My use case:

• top-app: depends on "react", dynamically load "external-lib"
• external-lib: depends on "react"

external-lib is matained and deployed by another team

In order to share "react" between "top-app" and "external-lib", I am forced to build react as a separate systemjs bundle:

• react.system.js
• top-app.system.js
• external-lib.system.js (deployed by another team)

Separating "react" lead to a loading waterfall:

1. html and inline bootstrap code(systemjs runtime and System.import("top-app.system.js"))
2. load top-app.system.js
3. load react.system.js`. Top-app can‘t do anything before this finish!
4. (when needed, dynamically load external-lib.system.js)

What I want is this:

1. html and inline bootstrap code: systemjs runtime and System.import('top-app.system.js'); System.import('react.system.js');.
2. load "top-app.system.js" and "react.system.js" in parallel. Top-app is functional after this finish
3. (when needed, dynamically load external-lib.system.js)

As you can see, I can use multiple import to achieve parallel loading, but depcache can make this scalable.

Another question: Seeing that "top-app.system.js" and "react.system.js" is critical resouce for the page, can I inline "top-app.system.js" and "react.system.js" into HTML? So that I can further reduce one request ?

1. html and inline bootstrap code: systemjs runtime and Inline "top-app.system.js" and "react.system.js". Top-app is functional after this finish
2. (when needed, dynamically load external-lib.system.js)

I ask this question in another issue: #2148 (comment)

Driving any spec response on depcache has completely stalled. But I'd still like to move towards making this a first-class SystemJS feature as it always was.

Does any one object to me merging this over the coming weeks? Any implementation concerns?

(I'm going to check the numbers again on some larger projects with the goal to merge)

I don't have any problems with this implementation

Released in 6.4.0.

@csr632
What is about your issue? Isn't it possible to include react to top-app and use System.register('react', ...) there to provide it to external-lib? Looks like it worked for me.