Add option to initialize mappings wasm via ArrayBuffer

[rtsao]

This PR adds the ability to directly pass in the mappings wasm into SourceMapConsumer.initialize. This is needed for browser usage without needing to make a request (e.g. needing to work around CSP restrictions).

Example

const contents = /* the ArrayBuffer contents of mappings.wasm */

sourceMap.SourceMapConsumer.initialize({
  "lib/mappings.wasm": contents
});

I didn't implement this because it would be a breaking change, but I think a better alternative API might be something like:

const wasmUrl = "https://unpkg.com/source-map@0.7.3/lib/mappings.wasm"; 

sourceMap.SourceMapConsumer.initialize({
  "lib/mappings.wasm": fetch(wasmUrl).then(res => res.arrayBuffer())
});

[coveralls]

Pull Request Test Coverage Report for Build 491

• 0 of 8 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 83.919%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/read-wasm.js	0	8	0.0%

Totals
Change from base Build 490:	-0.2%
Covered Lines:	908
Relevant Lines:	1051

---

💛 - Coveralls

[fitzgen]

Thanks!

[fitzgen]

Oh wait -- before we merge this, can you add a small test for initializing with an array buffer? Thanks!

[rtsao]

I was going to, but with the existing test tooling, there's no way to write a test for this because sourceMap.SourceMapConsumer.initialize is a no-op in a Node.js environment. In fact, currently, it appears all browser-specific logic is entirely untested.

Do you have an opinion regarding how browser testing should be done?

[fitzgen]

Bleh, I overlooked that. We would need some kind of headless browser testing setup. If you want to dig into adding that, please be my guest :)

In the meantime, we can just merge this. Thanks again!

[fitzgen]

Can you rebase this branch on master?

[rtsao]

Just did, thanks! 😄