Add diagnostic documentation for most of the ffi diagnostics

Change-Id: I37ba1852dea9c7e53966050eafaed9bb61cffbb8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/229580
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/src/test_utilities/mock_packages.dart

@@ -6,6 +6,33 @@ import 'package:analyzer/file_system/file_system.dart';
 
 /// Helper for creating mock packages.
 class MockPackages {
+  /// Create a fake 'ffi' package that can be used by tests.
+  static void addFfiPackageFiles(Folder rootFolder) {
+    var libFolder = rootFolder.getChildAssumingFolder('lib');
+    libFolder.getChildAssumingFile('ffi.dart').writeAsStringSync(r'''
+import 'dart:ffi';
+
+const Allocator calloc = _CallocAllocator();
+
+abstract class Allocator {
+  Pointer<T> allocate<T extends NativeType>(int byteCount, {int? alignment});
+
+  void free(Pointer pointer);
+}
+
+class Utf8 extends Opaque {}
+
+class _CallocAllocator implements Allocator {
+  @override
+  Pointer<T> allocate<T extends NativeType>(int byteCount, {int? alignment})
+      => throw '';
+
+  @override
+  void free(Pointer pointer) => throw '';
+}
+''');
+  }
+
   /// Create a fake 'js' package that can be used by tests.
   static void addJsPackageFiles(Folder rootFolder) {
     var libFolder = rootFolder.getChildAssumingFolder('lib');

pkg/analyzer/lib/src/test_utilities/mock_sdk.dart

@@ -662,6 +662,8 @@ class NativeType {
 
 class Handle extends NativeType {}
 
+abstract class Opaque extends NativeType {}
+
 class Void extends NativeType {}
 
 class Int8 extends NativeType {

pkg/analyzer/test/src/dart/resolution/context_collection_resolution.dart

@@ -313,6 +313,7 @@ class PubPackageResolutionTest extends ContextResolutionTest {
   void writeTestPackageConfig(
     PackageConfigFileBuilder config, {
     String? languageVersion,
+    bool ffi = false,
     bool js = false,
     bool meta = false,
   }) {
@@ -324,6 +325,14 @@ class PubPackageResolutionTest extends ContextResolutionTest {
       languageVersion: languageVersion ?? testPackageLanguageVersion,
     );
 
+    if (ffi) {
+      var ffiPath = '/packages/ffi';
+      MockPackages.addFfiPackageFiles(
+        getFolder(ffiPath),
+      );
+      config.add(name: 'ffi', rootPath: ffiPath);
+    }
+
     if (js) {
       var jsPath = '/packages/js';
       MockPackages.addJsPackageFiles(

pkg/analyzer/test/verify_diagnostics_test.dart

@@ -451,6 +451,6 @@ class _SnippetTest extends PubPackageResolutionTest {
         );
       }
     }
-    writeTestPackageConfig(packageConfigBuilder, meta: true);
+    writeTestPackageConfig(packageConfigBuilder, ffi: true, meta: true);
   }
 }