Add chat samples (#192)

Add samples with the standardized prompts for chatting with text and
images.

One difference from the canonical sample is that the image is uploaded
directly as a data part instead of uploaded as a file, since the Dart
SDK does not support uploading files.

Author: natebosch

Diff for: samples/dart/bin/chat.dart

@@ -0,0 +1,104 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'dart:io';
+
+import 'package:google_generative_ai/google_generative_ai.dart';
+
+final apiKey = () {
+  final apiKey = Platform.environment['GEMINI_API_KEY'];
+  if (apiKey == null) {
+    stderr.writeln(r'No $GEMINI_API_KEY environment variable');
+    exit(1);
+  }
+  return apiKey;
+}();
+
+Future<void> chat() async {
+  // [START chat]
+  final model = GenerativeModel(
+    model: 'gemini-1.5-flash',
+    apiKey: apiKey,
+  );
+  final chat = model.startChat(history: [
+    Content.text('hello'),
+    Content.model([TextPart('Great to meet you. What would you like to know?')])
+  ]);
+  var response =
+      await chat.sendMessage(Content.text('I have 2 dogs in my house.'));
+  print(response.text);
+  response =
+      await chat.sendMessage(Content.text('How many paws are in my house?'));
+  print(response.text);
+  // [END chat]
+}
+
+Future<void> chatStreaming() async {
+  // [START chat_streaming]
+  final model = GenerativeModel(
+    model: 'gemini-1.5-flash',
+    apiKey: apiKey,
+  );
+  final chat = model.startChat(history: [
+    Content.text('hello'),
+    Content.model([TextPart('Great to meet you. What would you like to know?')])
+  ]);
+  var responses =
+      chat.sendMessageStream(Content.text('I have 2 dogs in my house.'));
+  await for (final response in responses) {
+    print(response.text);
+    print('_' * 80);
+  }
+  responses =
+      chat.sendMessageStream(Content.text('How many paws are in my house?'));
+  await for (final response in responses) {
+    print(response.text);
+    print('_' * 80);
+  }
+  // [END chat_streaming]
+}
+
+Future<void> chatStreamingWithImages() async {
+  // [START chat_streaming_with_images]
+  final model = GenerativeModel(
+    model: 'gemini-1.5-flash',
+    apiKey: apiKey,
+  );
+  Future<DataPart> fileToPart(String mimeType, String path) async {
+    return DataPart(mimeType, await File(path).readAsBytes());
+  }
+
+  final chat = model.startChat();
+  var responses = chat.sendMessageStream(Content.text(
+      "Hello, I'm interested in learning about musical instruments. "
+      'Can I show you one?'));
+  await for (final response in responses) {
+    print(response.text);
+    print('_' * 80);
+  }
+  final prompt = 'What family of instruments does this belong to?';
+  final image = await fileToPart('image/jpeg', 'resources/organ.jpg');
+  responses = chat.sendMessageStream(Content.multi([TextPart(prompt), image]));
+  await for (final response in responses) {
+    print(response.text);
+    print('_' * 80);
+  }
+  // [END chat_streaming_with_images]
+}
+
+void main() async {
+  await chat();
+  await chatStreaming();
+  await chatStreamingWithImages();
+}