Remove hacks that make macOs, Linux, and Windows not fail. (#19187)

They should fail, because we don't have real support yet.

Also, make the debug override work in release builds, so that people
on those platforms have a workaround.

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

@@ -23,29 +23,46 @@ enum TargetPlatform {
 /// The [TargetPlatform] that matches the platform on which the framework is
 /// currently executing.
 ///
+/// This is the default value of [ThemeData.platform] (hence the name). Widgets
+/// from the material library should use [Theme.of] to determine the current
+/// platform, rather than using [defaultTargetPlatform]. However, if there is
+/// widget behavior that depends on the actual underlying platform (e.g. because
+/// it is controlling data being sent to the platform APIs, not just trying to
+/// follow the platform's conventions) then depending on [defaultTargetPlatform]
+/// makes sense.
+///
 /// In a test environment, the platform returned is [TargetPlatform.android]
 /// regardless of the host platform. (Android was chosen because the tests were
 /// originally written assuming Android-like behavior, and we added platform
 /// adaptations for iOS later). Tests can check iOS behavior by using the
 /// platform override APIs (such as [ThemeData.platform] in the material
-/// library) or by setting [debugDefaultTargetPlatformOverride]. The value
-/// can only be explicitly set in debug mode.
+/// library) or by setting [debugDefaultTargetPlatformOverride].
+//
+// When adding support for a new platform (e.g. Windows Phone, macOS), first
+// create a new value on the [TargetPlatform] enum, then add a rule for
+// selecting that platform here.
+//
+// It would be incorrect to make a platform that isn't supported by
+// [TargetPlatform] default to the behavior of another platform, because doing
+// that would mean we'd be stuck with that platform forever emulating the other,
+// and we'd never be able to introduce dedicated behavior for that platform
+// (since doing so would be a big breaking change).
 TargetPlatform get defaultTargetPlatform {
   TargetPlatform result;
-  if (Platform.isIOS || Platform.isMacOS) {
+  if (Platform.isIOS) {
     result = TargetPlatform.iOS;
-  } else if (Platform.isAndroid || Platform.isLinux || Platform.isWindows) {
+  } else if (Platform.isAndroid) {
     result = TargetPlatform.android;
-  } else if (Platform.operatingSystem == 'fuchsia') {
+  } else if (Platform.isFuchsia) {
     result = TargetPlatform.fuchsia;
   }
   assert(() {
     if (Platform.environment.containsKey('FLUTTER_TEST'))
       result = TargetPlatform.android;
-    if (debugDefaultTargetPlatformOverride != null)
-      result = debugDefaultTargetPlatformOverride;
     return true;
   }());
+  if (debugDefaultTargetPlatformOverride != null)
+    result = debugDefaultTargetPlatformOverride;
   if (result == null) {
     throw new FlutterError(
       'Unknown platform.\n'
@@ -55,12 +72,24 @@ TargetPlatform get defaultTargetPlatform {
   }
   return result;
 }
+
 /// Override the [defaultTargetPlatform].
 ///
 /// Setting this to null returns the [defaultTargetPlatform] to its original
 /// value (based on the actual current platform).
 ///
-/// This setter is only available intended for debugging purposes. To change the
-/// target platform in release builds, use the platform override APIs (such as
-/// [ThemeData.platform] in the material library).
+/// Generally speaking this override is only useful for tests. To change the
+/// platform that widgets resemble, consider using the platform override APIs
+/// (such as [ThemeData.platform] in the material library) instead.
+///
+/// Setting [debugDefaultTargetPlatformOverride] (as opposed to, say,
+/// [ThemeData.platform]) will cause unexpected and undesireable effects. For
+/// example, setting this to [TargetPlatform.iOS] when the application is
+/// running on Android will cause the TalkBack accessibility tool on Android to
+/// be confused because it would be receiving data intended for iOS VoiceOver.
+/// Similarly, setting it to [TargetPlatform.android] while on iOS will cause
+/// certainly widgets to work assuming the presence of a system-wide back
+/// button, which will make those widgets unusable since iOS has no such button.
+///
+/// In general, therefore, this property should not be used in release builds.
 TargetPlatform debugDefaultTargetPlatformOverride;

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

@@ -102,14 +102,14 @@ class Drawer extends StatelessWidget {
   /// {@macro flutter.widgets.child}
   final Widget child;
 
-  /// The semantic label of the dialog used by accessibility frameworks to 
+  /// The semantic label of the dialog used by accessibility frameworks to
   /// announce screen transitions when the drawer is opened and closed.
-  /// 
+  ///
   /// If this label is not provided, it will default to
   /// [MaterialLocalizations.drawerLabel].
-  /// 
+  ///
   /// See also:
-  /// 
+  ///
   ///  * [SemanticsConfiguration.namesRoute], for a description of how this
   ///    value is used.
   final String semanticLabel;

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

@@ -432,28 +432,17 @@ class TextTheme extends Diagnosticable {
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
     final TextTheme defaultTheme = new Typography(platform: defaultTargetPlatform).black;
-    properties.add(new DiagnosticsProperty<TextStyle>('display4', display4,
-        defaultValue: defaultTheme.display4));
-    properties.add(new DiagnosticsProperty<TextStyle>('display3', display3,
-        defaultValue: defaultTheme.display3));
-    properties.add(new DiagnosticsProperty<TextStyle>('display2', display2,
-        defaultValue: defaultTheme.display2));
-    properties.add(new DiagnosticsProperty<TextStyle>('display1', display1,
-        defaultValue: defaultTheme.display1));
-    properties.add(new DiagnosticsProperty<TextStyle>('headline', headline,
-        defaultValue: defaultTheme.headline));
-    properties
-        .add(new DiagnosticsProperty<TextStyle>('title', title, defaultValue: defaultTheme.title));
-    properties.add(
-        new DiagnosticsProperty<TextStyle>('subhead', subhead, defaultValue: defaultTheme.subhead));
-    properties
-        .add(new DiagnosticsProperty<TextStyle>('body2', body2, defaultValue: defaultTheme.body2));
-    properties
-        .add(new DiagnosticsProperty<TextStyle>('body1', body1, defaultValue: defaultTheme.body1));
-    properties.add(
-        new DiagnosticsProperty<TextStyle>('caption', caption, defaultValue: defaultTheme.caption));
-    properties.add(
-        new DiagnosticsProperty<TextStyle>('button', button, defaultValue: defaultTheme.button));
+    properties.add(new DiagnosticsProperty<TextStyle>('display4', display4, defaultValue: defaultTheme.display4));
+    properties.add(new DiagnosticsProperty<TextStyle>('display3', display3, defaultValue: defaultTheme.display3));
+    properties.add(new DiagnosticsProperty<TextStyle>('display2', display2, defaultValue: defaultTheme.display2));
+    properties.add(new DiagnosticsProperty<TextStyle>('display1', display1, defaultValue: defaultTheme.display1));
+    properties.add(new DiagnosticsProperty<TextStyle>('headline', headline, defaultValue: defaultTheme.headline));
+    properties.add(new DiagnosticsProperty<TextStyle>('title', title, defaultValue: defaultTheme.title));
+    properties.add(new DiagnosticsProperty<TextStyle>('subhead', subhead, defaultValue: defaultTheme.subhead));
+    properties.add(new DiagnosticsProperty<TextStyle>('body2', body2, defaultValue: defaultTheme.body2));
+    properties.add(new DiagnosticsProperty<TextStyle>('body1', body1, defaultValue: defaultTheme.body1));
+    properties.add(new DiagnosticsProperty<TextStyle>('caption', caption, defaultValue: defaultTheme.caption));
+    properties.add(new DiagnosticsProperty<TextStyle>('button', button, defaultValue: defaultTheme.button));
   }
 }
 

packages/flutter/lib/src/widgets/modal_barrier.dart

@@ -83,7 +83,7 @@ class ModalBarrier extends StatelessWidget {
               constraints: const BoxConstraints.expand(),
               child: color == null ? null : new DecoratedBox(
                 decoration: new BoxDecoration(
-                  color: color
+                  color: color,
                 )
               )
             )

packages/flutter_tools/lib/src/tester/flutter_tester.dart

@@ -151,9 +151,10 @@ class FlutterTesterDevice extends Device {
 
       _isRunning = true;
       _process = await processManager.start(command,
-      environment: <String, String>{
-        'FLUTTER_TEST': 'true',
-      });
+        environment: <String, String>{
+          'FLUTTER_TEST': 'true',
+        },
+      );
       _process.exitCode.then((_) => _isRunning = false);
       _process.stdout
           .transform(utf8.decoder)