Eliminate Dart 1 support from Xcode xcconfig files (#21400)

packages/flutter_tools/lib/src/ios/mac.dart

@@ -236,7 +236,6 @@ Future<XcodeBuildResult> buildXcodeProject({
   await updateGeneratedXcodeProperties(
     project: project,
     targetOverride: targetOverride,
-    previewDart2: true,
     buildInfo: buildInfo,
   );
 

packages/flutter_tools/lib/src/ios/xcodeproj.dart

@@ -34,7 +34,6 @@ Future<void> updateGeneratedXcodeProperties({
   @required FlutterProject project,
   @required BuildInfo buildInfo,
   String targetOverride,
-  @required bool previewDart2,
 }) async {
   final StringBuffer localsBuffer = new StringBuffer();
 
@@ -89,10 +88,6 @@ Future<void> updateGeneratedXcodeProperties({
     localsBuffer.writeln('ARCHS=$arch');
   }
 
-  if (previewDart2) {
-    localsBuffer.writeln('PREVIEW_DART_2=true');
-  }
-
   if (buildInfo.trackWidgetCreation) {
     localsBuffer.writeln('TRACK_WIDGET_CREATION=true');
   }

packages/flutter_tools/lib/src/project.dart

@@ -230,7 +230,6 @@ class IosProject {
         project: parent,
         buildInfo: BuildInfo.debug,
         targetOverride: bundle.defaultMainPath,
-        previewDart2: true,
       );
     }
   }

packages/flutter_tools/test/ios/xcodeproj_test.dart

@@ -290,7 +290,6 @@ Information about project "Runner":
       await updateGeneratedXcodeProperties(
         project: project,
         buildInfo: buildInfo,
-        previewDart2: true,
       );
 
       final File config = fs.file('path/to/project/ios/Flutter/Generated.xcconfig');
@@ -308,7 +307,6 @@ Information about project "Runner":
       await updateGeneratedXcodeProperties(
         project: project,
         buildInfo: buildInfo,
-        previewDart2: true,
       );
 
       final File config = fs.file('path/to/project/ios/Flutter/Generated.xcconfig');
@@ -326,7 +324,6 @@ Information about project "Runner":
       await updateGeneratedXcodeProperties(
         project: project,
         buildInfo: buildInfo,
-        previewDart2: true,
       );
 
       final File config = fs.file('path/to/project/ios/Flutter/Generated.xcconfig');
@@ -345,7 +342,6 @@ Information about project "Runner":
       await updateGeneratedXcodeProperties(
         project: project,
         buildInfo: buildInfo,
-        previewDart2: true,
       );
 
       final File config = fs.file('path/to/project/ios/Flutter/Generated.xcconfig');
@@ -383,7 +379,6 @@ Information about project "Runner":
       await updateGeneratedXcodeProperties(
         project: await FlutterProject.fromPath('path/to/project'),
         buildInfo: buildInfo,
-        previewDart2: true,
       );
 
       final File localPropertiesFile = fs.file('path/to/project/ios/Flutter/Generated.xcconfig');