Explore emitting an mjs file for each deferred unit

[biggs0125]

Some helper code that lives in the mjs file produced by dart2wasm may be considered deferrable using the deferred import graph.

Things that might be deferred:

• JS interop wrappers
• Non-utf-8 strings

When one of these values is only used from a deferred wasm module, we can emit an mjs module as well that exposes that data. This will reduce the size of the main mjs file thereby improving page load times.

[biggs0125]

cc @mkustermann