[dart2wasm] Clarify run instructions

Change-Id: I693a1adf3c028656864014f7c41bad481bb8b106
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/271881
Reviewed-by: Michael Thomsen <[email protected]>
Commit-Queue: Aske Simon Christensen <[email protected]>

Author: askeksa

pkg/dart2wasm/dart2wasm.md

@@ -1,8 +1,8 @@
 ## Running dart2wasm
 
-You don't need to build the Dart SDK to run dart2wasm, as long as you have a Dart SDK installed.
+You don't need to build the Dart SDK to run dart2wasm, as long as you have a Dart SDK installed and have the [Dart SDK repository checked out](https://github.com/dart-lang/sdk/wiki/Building#getting-the-source).
 
-To compile a Dart file to Wasm, run:
+To compile a Dart file to Wasm, in a checkout of the Dart SDK repository, run:
 
 `dart --enable-asserts pkg/dart2wasm/bin/dart2wasm.dart` *options* *infile*`.dart` *outfile*`.wasm`