Document how to test after making changes to Fasta.

pkg/front_end/lib/src/fasta/README.md

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 # Fasta -- Fully-resolved AST, Accelerated.
 
 Fasta is a compiler framework for compiling Dart sources to Kernel IR. When Fasta works well, you won't even know you're using it, as it will be transparently integrated in tools like *dart*, *dartanalyzer*, *dart2js*, etc.
 
 Hopefully, you'll notice that Fasta-based tools are fast, with good error messages. If not, please let us [know](https://github.com/dart-lang/sdk/issues/new).
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 This will generate `pkg/front_end/test/fasta/hello.dart.dill` which can be run this way:
 
 ```bash
 $DART_AOT_VM pkg/front_end/test/fasta/hello.dart.dill
 ```
 
 Where `$DART_AOT_VM` is defined as described [above](#Building-The-Dart-SDK).
 
 ## Running Tests
 
-Run:
-
-```bash
-dart -c pkg/testing/bin/testing.dart --config=pkg/front_end/test/fasta/testing.json
-```
+See [How to test Fasta](TESTING.md)
 
 ## Running dart2js
 
 ```bash
 dart pkg/front_end/lib/src/fasta/bin/compile.dart pkg/compiler/lib/src/dart2js.dart
 $DART_AOT_VM pkg/compiler/lib/src/dart2js.dart.dill pkg/front_end/test/fasta/hello.dart
 ```
 
 The output of dart2js will be `out.js`, and it can be run on any Javascript engine, for example, d8 which is included with the Dart SDK sources:
 
 ```
 ./third_party/d8/<OS>/d8 out.js
 ```
 
 Where `<OS>` is one of `linux`, `macos`, or `windows`.