Issue 1:
I'm attempting to bundle a React app with deno and am running into a strange backtrace if I run it with an empty cache.

alexmax@Alexs-iMac env.ts % uname -a
Darwin Alexs-iMac.local 18.7.0 Darwin Kernel Version 18.7.0: Sun Dec 1 18:59:03 PST 2019; root:xnu-4903.278.19~1/RELEASE_X86_64 x86_64
alexmax@Alexs-iMac env.ts % deno --version
deno 0.30.0
v8 8.1.108
typescript 3.7.2
alexmax@Alexs-iMac env.ts % RUST_BACKTRACE=1 deno bundle index.ts
Download https://dev.jspm.io/react
Download https://dev.jspm.io/react-dom
Download https://dev.jspm.io/npm:react@16.12.0/index.dew.js
Download https://dev.jspm.io/npm:react@16.12.0/cjs/react.development.dew.js
Download https://dev.jspm.io/npm:object-assign@4?dew
Download https://dev.jspm.io/npm:prop-types@15/checkPropTypes?dew
Download https://dev.jspm.io/npm:object-assign@4.1.1/index.dew.js
Download https://dev.jspm.io/npm:prop-types@15.7.2/checkPropTypes.dew.js
Download https://dev.jspm.io/npm:prop-types@15.7.2/lib/ReactPropTypesSecret.dew.js
Download https://dev.jspm.io/npm:react-dom@16.12.0/index.dew.js
Download https://dev.jspm.io/npm:react-dom@16.12.0/cjs/react-dom.development.dew.js
Download https://dev.jspm.io/npm:react@16?dew
Download https://dev.jspm.io/npm:scheduler@0.18?dew
Download https://dev.jspm.io/npm:scheduler@0.18/tracing?dew
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 17, kind: AlreadyExists, message: "File exists" }', src/libcore/result.rs:1165:5
stack backtrace:
0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
1: core::fmt::write
2: std::io::Write::write_fmt
3: std::panicking::default_hook::{{closure}}
4: std::panicking::default_hook
5: std::panicking::rust_panic_with_hook
6: std::panicking::continue_panic_fmt
7: rust_begin_unwind
8: core::panicking::panic_fmt
9: core::result::unwrap_failed
10: <std::future::GenFuture<T> as core::future::future::Future>::poll
11: <std::future::GenFuture<T> as core::future::future::Future>::poll
12: <futures_util::future::try_join_all::TryJoinAll<F> as core::future::future::Future>::poll
13: <std::future::GenFuture<T> as core::future::future::Future>::poll
14: futures_util::future::future::chain::Chain<Fut1,Fut2,Data>::poll
15: <futures_util::future::future::then::Then<Fut1,Fut2,F> as core::future::future::Future>::poll
16: <futures_util::future::try_future::map_ok::MapOk<Fut,F> as core::future::future::Future>::poll
17: <futures_util::future::try_future::map_ok::MapOk<Fut,F> as core::future::future::Future>::poll
18: futures_util::stream::stream::StreamExt::poll_next_unpin
19: <deno_core::isolate::Isolate as core::future::future::Future>::poll
20: <deno_core::es_isolate::EsIsolate as core::future::future::Future>::poll
21: <deno::worker::Worker as core::future::future::Future>::poll
22: <std::future::GenFuture<T> as core::future::future::Future>::poll
23: tokio::runtime::enter::Enter::block_on
24: tokio::runtime::context::enter
25: deno::tokio_util::run
26: deno::main
27: std::rt::lang_start::{{closure}}
28: std::panicking::try::do_call
29: __rust_maybe_catch_panic
30: std::rt::lang_start_internal
31: main

If I then run the command again immediately, I don't get a Rust exception, but I get a normal pretty exception.

alexmax@Alexs-iMac env.ts % RUST_BACKTRACE=1 deno bundle index.ts
Download https://dev.jspm.io/npm:react-dom@16.12.0/cjs/react-dom.development.dew.js
Download https://dev.jspm.io/npm:react@16?dew

error: Uncaught Other: Not a directory (os error 20)
► $deno$/dispatch_json.ts:40:11
at DenoError ($deno$/errors.ts:20:5)
at unwrapResponse ($deno$/dispatch_json.ts:40:11)
at sendAsync ($deno$/dispatch_json.ts:91:10)

Unfortunately, neither exception appears to give me any details as to what file path it's having issues with, or why it's having an issue with the file path in the first place.

Issue 2:
deno fetch https://dev.jspm.io/react-dom
Download https://dev.jspm.io/react-dom
Download https://dev.jspm.io/npm:react-dom@16.9.0/index.dew.js
Download https://dev.jspm.io/npm:react-dom@16.9.0/cjs/react-dom.development.dew.js
Download https://dev.jspm.io/npm:react@16?dew
Download https://dev.jspm.io/npm:object-assign@4?dew
Download https://dev.jspm.io/npm:prop-types@15/checkPropTypes?dew
Download https://dev.jspm.io/npm:scheduler@0.15?dew
Download https://dev.jspm.io/npm:scheduler@0.15/tracing?dew
Download https://dev.jspm.io/npm:object-assign@4.1.1/index.dew.js
Download https://dev.jspm.io/npm:prop-types@15.7.2/checkPropTypes.dew.js
Download https://dev.jspm.io/npm:prop-types@15.7.2/lib/ReactPropTypesSecret.dew.js
Download https://dev.jspm.io/npm:scheduler@0.15.0/tracing.dew.js
Download https://dev.jspm.io/npm:scheduler@0.15.0/cjs/scheduler-tracing.development.dew.js
thread 'tokio-runtime-worker-6' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 21, kind: Other, message: "Is a directory" }', src/libcore/result.rs:999:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
thread 'main' panicked at 'internal error: entered unreachable code', ../../third_party/rust_crates/registry/src/github.com-1ecc6299db9ec823/tokio-0.1.22/src/runtime/threadpool/mod.rs:296:26
thread 'tokio-runtime-worker-0' panicked at 'unexpected state while aborting task: Complete', ../../third_party/rust_crates/registry/src/github.com-1ecc6299db9ec823/tokio-threadpool-0.1.15/src/task/mod.rs:216:21

I don't know precisely what is going on, but something goes wrong..

Content of ~/Library/Caches/deno/deps/https/dev.jspm.io/npm:scheduler@0.15.headers.json is:

{"mime_type":"application/javascript"}

but ~/Library/Caches/deno/deps/https/dev.jspm.io/npm:scheduler@0.15 is DIRECTORY

This may occur when there are two url modules like these:

https://dev.jspm.io/npm:scheduler@0.15 <- application/javascript

https://dev.jspm.io/npm:scheduler@0.15/tracing <-application/javascript