Move dart2wasm support to be "normal"

[kevmoo]

At the moment there is an experimental-chrome-wasm flag, etc.

Ideally, Wasm support should just be like dart2js support. Wasm tests should "just pass" on (latest) Chrome and Firefox.

We need to test the following packages with wasm

• sse - Test with dart2wasm compilation tools#1654
• http - Test with dart2wasm compilation http#1056
• web_socket_channel - Test with dart2wasm compilation http#1653

CC @jakemac53 @srujzs