chore: Refine Dart config parsing

We are experimenting with some new config layouts and the current parsing behavior prevents changing the config structure. This expands on the current behavior to allow more flexibility in the
config file contents.

packages/amplify-frontend-flutter/lib/dart-fs.js

@@ -6,16 +6,23 @@ function readJsonFromDart(jsonFilePath, encoding = 'utf8', throwOnError = true)
     return undefined;
   }
   const fileContents = fs.readFileSync(jsonFilePath, encoding);
-  let jsonValue = fileContents.substring(fileContents.indexOf('{'), fileContents.lastIndexOf('}') + 1);
+  const configStart = fileContents.indexOf('const amplifyconfig');
+  if (configStart === -1) {
+    return undefined;
+  }
+  const QUOTE = "'''";
+  const jsonStart = fileContents.indexOf(QUOTE, configStart) + QUOTE.length;
+  const jsonEnd = fileContents.indexOf(QUOTE, jsonStart);
+  const jsonValue = fileContents.substring(jsonStart, jsonEnd).trim();
   return JSON.parse(jsonValue);
 }
 
-function writeJsonToDart(dest, obj) {
+function writeJsonToDart(dest, json) {
   const destPath = path.parse(dest).dir;
   if (!fs.existsSync(destPath)) {
     fs.mkdirSync(destPath, { recursive: true });
   }
-  const dartContent = `const amplifyconfig = ''' ${obj}''';`;
+  const dartContent = `const amplifyconfig = '''${json}''';\n`;
   fs.writeFileSync(dest, dartContent);
 }