[doc] dart:ffi SQLite sample

Issue: dart-lang/sdk#35775

Change-Id: I2ce86c554ffd6f49050cf63ead60809c08fb02e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97504
Reviewed-by: Michael Thomsen <mit@google.com>

samples/ffi/sqlite/.gitignore

@@ -0,0 +1,7 @@
+.dart_tool
+.gdb_history
+.packages
+.vscode
+pubspec.lock
+test.db
+test.db-journal

samples/ffi/sqlite/README.md

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+# Sample code dart:ffi
+
+This is an illustrative sample for how to use `dart:ffi`.
+
+
+## Building and Running this Sample
+
+Building and running this sample is done through pub.
+Running `pub get` and `pub run test` should produce the following output.
+
+```sh
+$ pub get
+Resolving dependencies... (6.8s)
++ analyzer 0.35.4
+...
++ yaml 2.1.15
+Downloading analyzer 0.35.4...
+Downloading kernel 0.3.14...
+Downloading front_end 0.1.14...
+Changed 47 dependencies!
+Precompiling executables... (18.0s)
+Precompiled test:test.
+
+```
+
+```
+$ pub run test
+00:01 +0: test/sqlite_test.dart: sqlite integration test                                                                                                     
+1 Chocolade chip cookie Chocolade cookie foo
+2 Ginger cookie null 42
+3 Cinnamon roll null null
+1 Chocolade chip cookie Chocolade cookie foo
+2 Ginger cookie null 42
+expected exception on accessing result data after close: The result has already been closed.
+expected this query to fail: no such column: non_existing_column (Code 1: SQL logic error)
+00:02 +3: All tests passed! 
+```
+
+## Tutorial
+
+A tutorial walking through the code is available in [docs/sqlite-tutorial.md](docs/sqlite-tutorial.md).