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

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

 // TODO(sigmund): rename universe => world
 /// Describes individual features that may be seen in a program. Most features
 /// can be described only by name using the [Feature] enum, some features are
 /// expressed including details on how they are used. For example, whether a
 /// list literal was constant or empty.
 ///
 /// The use of these features is typically discovered in an early phase of the
 /// compilation pipeline, for example during resolution.
 library compiler.universe.feature;
 
@@ skipping 60 matching lines @@
   /// A method with a `sync*` body modifier.
   SYNC_STAR,
 
   /// A throw expression.
   THROW_EXPRESSION,
 
   /// An implicit throw of a `NoSuchMethodError`, like calling an unresolved
   /// 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,
 }
 
 /// Describes a use of a map literal in the program.
 class MapLiteralUse {
   final InterfaceType type;
   final bool isConstant;
   final bool isEmpty;
@@ skipping 38 matching lines @@
     if (other is! ListLiteralUse) return false;
     return type == other.type &&
         isConstant == other.isConstant &&
         isEmpty == other.isEmpty;
   }
 
   String toString() {
     return 'ListLiteralUse($type,isConstant:$isConstant,isEmpty:$isEmpty)';
   }
 }