Don't crash when pubspec isn't a map (#59632)

packages/flutter_tools/lib/src/flutter_manifest.dart

@@ -40,16 +40,16 @@ class FlutterManifest {
   /// Returns null on missing or invalid manifest
   @visibleForTesting
   static FlutterManifest createFromString(String manifest, { @required Logger logger }) {
-    return _createFromYaml(loadYaml(manifest) as YamlMap, logger);
+    return _createFromYaml(manifest != null ? loadYaml(manifest) : null, logger);
   }
 
-  static FlutterManifest _createFromYaml(YamlMap yamlDocument, Logger logger) {
-    final FlutterManifest pubspec = FlutterManifest._(logger);
+  static FlutterManifest _createFromYaml(dynamic yamlDocument, Logger logger) {
     if (yamlDocument != null && !_validate(yamlDocument, logger)) {
       return null;
     }
 
-    final Map<dynamic, dynamic> yamlMap = yamlDocument;
+    final FlutterManifest pubspec = FlutterManifest._(logger);
+    final Map<dynamic, dynamic> yamlMap = yamlDocument as YamlMap;
     if (yamlMap != null) {
       pubspec._descriptor = yamlMap.cast<String, dynamic>();
     } else {
@@ -332,9 +332,12 @@ String buildSchemaPath(FileSystem fileSystem) {
 /// This method should be kept in sync with the schema in
 /// `$FLUTTER_ROOT/packages/flutter_tools/schema/pubspec_yaml.json`,
 /// but avoid introducing dependencies on packages for simple validation.
-bool _validate(YamlMap manifest, Logger logger) {
+bool _validate(dynamic manifest, Logger logger) {
   final List<String> errors = <String>[];
-  for (final MapEntry<dynamic, dynamic> kvp in manifest.entries) {
+  if (manifest is! YamlMap) {
+    errors.add('Expected YAML map');
+  } else {
+    for (final MapEntry<dynamic, dynamic> kvp in (manifest as YamlMap).entries) {
       if (kvp.key is! String) {
         errors.add('Expected YAML key to be a string, but got ${kvp.key}.');
         continue;
@@ -360,6 +363,7 @@ bool _validate(YamlMap manifest, Logger logger) {
           break;
       }
     }
+  }
 
   if (errors.isNotEmpty) {
     logger.printStatus('Error detected in pubspec.yaml:', emphasis: true);

packages/flutter_tools/test/general.shard/flutter_manifest_test.dart

@@ -32,6 +32,16 @@ void main() {
     expect(flutterManifest.assets, isEmpty);
   });
 
+  testWithoutContext('FlutterManifest is null when the pubspec.yaml file is not a map', () async {
+    final BufferLogger logger = BufferLogger.test();
+    expect(FlutterManifest.createFromString(
+      'Not a map',
+      logger: logger,
+    ), isNull);
+
+    expect(logger.errorText, contains('Expected YAML map'));
+  });
+
   testWithoutContext('FlutterManifest has no fonts or assets when the "flutter" section is empty', () async {
     const String manifest = '''
 name: test