Remove factory body in *.fromEnvironment, and implement this same behavior

pkg/compiler/lib/src/universe/feature.dart

Index: pkg/compiler/lib/src/universe/feature.dart
diff --git a/pkg/compiler/lib/src/universe/feature.dart b/pkg/compiler/lib/src/universe/feature.dart
index 03315f070cba1c5960bd93de5925f982ce70de0d..d9799bca13e015aa4513f257fa48d4a1ecbb35fe 100644
--- a/pkg/compiler/lib/src/universe/feature.dart
+++ b/pkg/compiler/lib/src/universe/feature.dart
@@ -78,9 +78,13 @@ enum Feature {
   /// static method.
   THROW_NO_SUCH_METHOD,
 
-  /// An implicit throw of a runtime error, like
+  /// An implicit throw of a runtime error, like in a runtime type check.
   THROW_RUNTIME_ERROR,
 
+  /// An implicit throw of a `UnsupportedError`, like calling `new
+  /// bool.fromEnvironment`.
+  THROW_UNSUPPORTED_ERROR,
+
   /// The need for a type variable bound check, like instantiation of a generic
   /// type whose type variable have non-trivial bounds.
   TYPE_VARIABLE_BOUNDS_CHECK,