Be more explicit about the default TargetPlatform (#5034)

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

@@ -2,6 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io' show Platform;
+
+import 'assertions.dart';
+
 /// The platform that user interaction should adapt to target.
 enum TargetPlatform {
   /// Android: <https://www.android.com/>
@@ -10,3 +14,16 @@ enum TargetPlatform {
   /// iOS: <http://www.apple.com/ios/>
   iOS,
 }
+
+/// The [TargetPlatform] that matches the platform on which the framework is currently executing.
+TargetPlatform get defaultTargetPlatform {
+  if (Platform.isIOS || Platform.isMacOS)
+    return TargetPlatform.iOS;
+  if (Platform.isAndroid || Platform.isLinux)
+    return TargetPlatform.android;
+  throw new FlutterError(
+    'Unknown platform\n'
+    '${Platform.operatingSystem} was not recognized as a target platform. '
+    'Consider updating the list of TargetPlatforms to include this platform.'
+  );
+}

packages/flutter/lib/src/material/theme_data.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io' show Platform;
 import 'dart:ui' show Color, hashValues;
 
 import 'package:flutter/foundation.dart';
@@ -125,7 +124,7 @@ class ThemeData {
     primaryTextTheme ??= primaryIsDark ? Typography.white : Typography.black;
     iconTheme ??= isDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
     primaryIconTheme ??= primaryIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
-    platform ??= (Platform.isIOS || Platform.isMacOS) ? TargetPlatform.iOS : TargetPlatform.android;
+    platform ??= defaultTargetPlatform;
     return new ThemeData.raw(
       brightness: brightness,
       primaryColor: primaryColor,