Suppress undefined_getter error when analyzing colors.dart with an older SDK.

pkg/front_end/lib/src/fasta/colors.dart

Index: pkg/front_end/lib/src/fasta/colors.dart
diff --git a/pkg/front_end/lib/src/fasta/colors.dart b/pkg/front_end/lib/src/fasta/colors.dart
index 301e9ad281c23d22756107d461da585ef8cc1c83..649229f65bce29cf7fd660b50e2bf2e537c01f6e 100644
--- a/pkg/front_end/lib/src/fasta/colors.dart
+++ b/pkg/front_end/lib/src/fasta/colors.dart
@@ -100,6 +100,7 @@ String white(String string) => wrap(string, WHITE_COLOR);
 bool computeEnableColors(CompilerContext context) {
   bool ansiSupported;
   try {
+    // ignore: undefined_getter
     ansiSupported = Platform.ansiSupported;
   } on NoSuchMethodError {
     // Ignored: We're running on an older version of the Dart VM which doesn't