Issue 1:
Errors for use of non stable APIs are non-obvious. For example:

$ deno run --allow-read --allow-write main.ts
Compile file:///dev/deno/main.ts
Download https://deno.land/std@0.50.0/fs/copy.ts
Download https://deno.land/std@0.50.0/fs/ensure_dir.ts
Download https://deno.land/std@0.50.0/fs/_util.ts
error: TS2339 [ERROR]: Property 'utime' does not exist on type 'typeof Deno'.
await Deno.utime(dest, statInfo.atime, statInfo.mtime);
~~~~~
at https://deno.land/std@0.50.0/fs/copy.ts:90:16

Is there a way that we could include the unstable types at runtime where they could be checked (but not used) when errors of type Property xyz does not exist... occur to provide a more helpful error message?

That would allow us to output something like:

$ deno run --allow-read --allow-write main.ts
Compile file:///dev/deno/main.ts
Download https://deno.land/std@0.50.0/fs/copy.ts
Download https://deno.land/std@0.50.0/fs/ensure_dir.ts
Download https://deno.land/std@0.50.0/fs/_util.ts
error: TS2339 [ERROR]: Property 'utime' does not exist on type 'typeof Deno'.
'utime' is an unstable property of Deno. Add the --unstable flag to enable use.
await Deno.utime(dest, statInfo.atime, statInfo.mtime);
~~~~~
at https://deno.land/std@0.50.0/fs/copy.ts:90:16

Issue 2:
I'm trying the following script and I get a few errors.

index.ts

import { exists } from "https://deno.land/std/fs/mod.ts";

const found = await exists('/folder');
console.log('Found:', found);

Error:

error TS2339: Property 'utime' does not exist on type 'typeof Deno'.
await Deno.utime(dest, statInfo.atime, statInfo.mtime);
~~~~~
at https://deno.land/std/fs/copy.ts:90:16

error TS2339: Property 'utimeSync' does not exist on type 'typeof Deno'.
Deno.utimeSync(dest, statInfo.atime, statInfo.mtime);
~~~~~~~~~
at https://deno.land/std/fs/copy.ts:101:10

error TS2339: Property 'symlink' does not exist on type 'typeof Deno'.
await Deno.symlink(originSrcFilePath, dest, type);
~~~~~~~
at https://deno.land/std/fs/copy.ts:114:14

error TS2339: Property 'utime' does not exist on type 'typeof Deno'.
await Deno.utime(dest, statInfo.atime, statInfo.mtime);
~~~~~
at https://deno.land/std/fs/copy.ts:119:16

error TS2339: Property 'symlinkSync' does not exist on type 'typeof Deno'.
Deno.symlinkSync(originSrcFilePath, dest, type);
~~~~~~~~~~~
at https://deno.land/std/fs/copy.ts:132:8

error TS2339: Property 'utimeSync' does not exist on type 'typeof Deno'.
Deno.utimeSync(dest, statInfo.atime, statInfo.mtime);
~~~~~~~~~
at https://deno.land/std/fs/copy.ts:137:10

error TS2339: Property 'utime' does not exist on type 'typeof Deno'.
await Deno.utime(dest, srcStatInfo.atime, srcStatInfo.mtime);
~~~~~
at https://deno.land/std/fs/copy.ts:157:16

error TS2339: Property 'utimeSync' does not exist on type 'typeof Deno'.
Deno.utimeSync(dest, srcStatInfo.atime, srcStatInfo.mtime);
~~~~~~~~~
at https://deno.land/std/fs/copy.ts:185:10

error TS2339: Property 'link' does not exist on type 'typeof Deno'.
await Deno.link(src, dest);
~~~~
at https://deno.land/std/fs/ensure_link.ts:28:14

error TS2339: Property 'linkSync' does not exist on type 'typeof Deno'.
Deno.linkSync(src, dest);
~~~~~~~~
at https://deno.land/std/fs/ensure_link.ts:52:8

error TS2339: Property 'symlink' does not exist on type 'typeof Deno'.
await Deno.symlink(src, dest, srcFilePathType);
~~~~~~~
at https://deno.land/std/fs/ensure_symlink.ts:31:14

error TS2339: Property 'symlinkSync' does not exist on type 'typeof Deno'.
Deno.symlinkSync(src, dest, srcFilePathType);
~~~~~~~~~~~
at https://deno.land/std/fs/ensure_symlink.ts:58:8

Found 12 errors.

deno 1.0.0-rc1

v8 8.2.308

typescript 3.8.3