Attempt to fix flutter drive --preview-dart-2 errors. (#14897)

packages/flutter/lib/src/foundation/binding.dart

@@ -20,7 +20,7 @@ import 'platform.dart';
 /// "type" key will be set to the string `_extensionType` to indicate
 /// that this is a return value from a service extension, and the
 /// "method" key will be set to the full name of the method.
-typedef Future<Map<String, String>> ServiceExtensionCallback(Map<String, String> parameters);
+typedef Future<Map<String, dynamic>> ServiceExtensionCallback(Map<String, String> parameters);
 
 /// Base class for mixins that provide singleton services (also known as
 /// "bindings").
@@ -135,7 +135,7 @@ abstract class BindingBase {
             }
             await reassembleApplication();
           }
-          return <String, String>{
+          return <String, dynamic>{
             'value': defaultTargetPlatform
                      .toString()
                      .substring('$TargetPlatform.'.length),
@@ -248,7 +248,7 @@ abstract class BindingBase {
       name: name,
       callback: (Map<String, String> parameters) async {
         await callback();
-        return <String, String>{};
+        return <String, dynamic>{};
       }
     );
   }
@@ -279,7 +279,7 @@ abstract class BindingBase {
       callback: (Map<String, String> parameters) async {
         if (parameters.containsKey('enabled'))
           await setter(parameters['enabled'] == 'true');
-        return <String, String>{ 'enabled': await getter() ? 'true' : 'false' };
+        return <String, dynamic>{ 'enabled': await getter() ? 'true' : 'false' };
       }
     );
   }
@@ -309,7 +309,7 @@ abstract class BindingBase {
       callback: (Map<String, String> parameters) async {
         if (parameters.containsKey(name))
           await setter(double.parse(parameters[name]));
-        return <String, String>{ name: (await getter()).toString() };
+        return <String, dynamic>{ name: (await getter()).toString() };
       }
     );
   }
@@ -338,7 +338,7 @@ abstract class BindingBase {
       callback: (Map<String, String> parameters) async {
         if (parameters.containsKey('value'))
           await setter(parameters['value']);
-        return <String, String>{ 'value': await getter() };
+        return <String, dynamic>{ 'value': await getter() };
       }
     );
   }
@@ -365,7 +365,7 @@ abstract class BindingBase {
       assert(method == methodName);
       dynamic caughtException;
       StackTrace caughtStack;
-      Map<String, String> result;
+      Map<String, dynamic> result;
       try {
         result = await callback(parameters);
       } catch (exception, stack) {