Port dom, fetch, guide-supported-types-examples, import_js examples to Rust 2018 edition

[T5uku5hi]

This PR ports the following examples:

• dom
• fetch
• guide-supported-types-examples
• import_js

I run these command.

cargo fix --edition
cargo check --target wasm32-unknown-unknown
cargo fix --edition-idioms

Relates to #1099.

🐞 bug report

When I run $ cargo fix --edition-idioms in import_js, I got an error message.
full output:

import_js $ cargo fix --edition-idioms
    Checking import_js v0.1.0 (/Users/misakimakino/Works/clones/fork-wasm-bindgen/wasm-bindgen/examples/import_js)
warning: failed to automatically apply fixes suggested by rustc to crate `import_js`

after fixes were automatically applied the compiler reported errors within these files:

  * examples/import_js/src/lib.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/cargo/issues
quoting the full output of this command we'd be very appreciative!

warning: `extern crate` is not idiomatic in the new edition
 --> examples/import_js/src/lib.rs:1:1
  |
1 | extern crate wasm_bindgen;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
  |
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`

warning: `extern crate` is not idiomatic in the new edition
 --> examples/import_js/src/lib.rs:1:1
  |
1 | extern crate wasm_bindgen;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`
  |
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`

    Finished dev [unoptimized + debuginfo] target(s) in 1.05s
import_js $

[fitzgen]

bug report

When I run $ cargo fix --edition-idioms in import_js, I got an error message.
full output:

FWIW, it is probably best to file this upstream in cargo's issue tracker!

[fitzgen]

LGTM with the little nitpicks below addressed :)

[fitzgen]

This line shouldn't be necessary in 2018 edition either :)

[fitzgen]

Ditto here and elsewhere. In general, we don't need use some_crate; in order to do some_crate::Something.

[T5uku5hi]

@fitzgen
Thank you for your comment.
I removed unnecessary lines.

[alexcrichton]

If you're extra ambitious, we can actually remove serde_derive both here and in Cargo.toml by adding this to Cargo.toml:

[dependencies]
serde = { version = "1.0.80", features = ["derive"] }

and then adding here:

use serde::{Deserialize, Serialize};

[T5uku5hi]

@alexcrichton
Thank you for your comment.
I'll try it !!

[T5uku5hi]

I added serde = { version = "1.0.80", features = ["derive"] } in Cargo.toml.

[alexcrichton]

👍

Thanks!