If you tried to implement a new WASM backend for Roslyn or javac or something, it would be impossible simply because of how much the behaviour of these languages depends on their VM. Stuff like `synchronized` and introspection (esp in CLR managed classes / reflection APIs). You need the VM for that. Projects like Scala Native work because you still have an entire OS to work with on the target platforms.
You would have to implement very low level virtual-machine-like features in WASM to support something like that. And even if you did, the scope of WASM isn't big enough to support all the features of something like Silverlight running in it. I don't think they will ever support a graphics API you could do that in, for example.
At a higher level, trying to run a heavy framework on top of WASM is just silly. You already have a heavy framework, in the DOM and other browser APIs. They are essentially all you could possibly need to do anything useful in a browser. They are the only way to display anything to the user. You can't run an HTTP server inside WASM, but you wouldn't want to. You can't implement a custom interface technology in WASM, because you can't draw anything, but you can do all that with the DOM APIs or WebGL/canvas/etc so why would you want to? All these things you would need to hack together a Silverlight are already available, and your wasm would need to include an implementation of all of them. These are all things you kinda want to dynamically link against... which wasm doesn't support...
The focus for WASM (after it properly nails down the spec on a whole lot of basics) will be memory management and enabling access to GC'd JS objects. Then using JS APIs would follow. We're a long way off both of those, let alone providing VM-like features you could target a managed language at, or enough native-like APIs to allow building something like Silverlight on top of it.
I say we, but it's definitely not me working on this stuff, and it's not half the engineering talent from Microsoft either.