fix type errors (#45151)

packages/flutter_tools/bin/fuchsia_tester.dart

@@ -134,7 +134,7 @@ Future<void> run(List<String> args) async {
 
     final Map<String, String> tests = <String, String>{};
     final List<Map<String, dynamic>> jsonList = List<Map<String, dynamic>>.from(
-      json.decode(fs.file(argResults[_kOptionTests]).readAsStringSync()) as List<Map<String, dynamic>>);
+      (json.decode(fs.file(argResults[_kOptionTests]).readAsStringSync()) as List<dynamic>).cast<Map<String, dynamic>>());
     for (Map<String, dynamic> map in jsonList) {
       final String source = fs.file(map['source']).resolveSymbolicLinksSync();
       final String dill = fs.file(map['dill']).resolveSymbolicLinksSync();

packages/flutter_tools/lib/src/tracing.dart

@@ -96,7 +96,7 @@ Future<void> downloadStartupTrace(VMService observatory, { bool awaitFirstFrame
 
   int extractInstantEventTimestamp(String eventName) {
     final List<Map<String, dynamic>> events =
-        List<Map<String, dynamic>>.from(timeline['traceEvents'] as List<Map<String, dynamic>>);
+        List<Map<String, dynamic>>.from((timeline['traceEvents'] as List<dynamic>).cast<Map<String, dynamic>>());
     final Map<String, dynamic> event = events.firstWhere(
       (Map<String, dynamic> event) => event['name'] == eventName, orElse: () => null,
     );

packages/flutter_tools/lib/src/vmservice_record_replay.dart

@@ -205,7 +205,9 @@ class ReplayVMServiceChannel extends StreamChannelMixin<String> {
   static Map<int, _Transaction> _loadTransactions(Directory location) {
     final File file = _getManifest(location);
     final String jsonData = file.readAsStringSync();
-    final Iterable<_Message> messages = (json.decode(jsonData) as List<Map<String, dynamic>>).map<_Message>(_toMessage);
+    final Iterable<_Message> messages = (json.decode(jsonData) as List<dynamic>)
+      .cast<Map<String, dynamic>>()
+      .map<_Message>(_toMessage);
     final Map<int, _Transaction> transactions = <int, _Transaction>{};
     for (_Message message in messages) {
       final _Transaction transaction =