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

pkg/compiler/lib/src/elements/entities.dart

Index: pkg/compiler/lib/src/elements/entities.dart
diff --git a/pkg/compiler/lib/src/elements/entities.dart b/pkg/compiler/lib/src/elements/entities.dart
index eedc0f65841a37aa6dcbd78733b3c3603a58d257..29eed5ce3f1d44b0ad38a41e9d5db3b2d8751bfa 100644
--- a/pkg/compiler/lib/src/elements/entities.dart
+++ b/pkg/compiler/lib/src/elements/entities.dart
@@ -137,6 +137,10 @@ abstract class ConstructorEntity extends FunctionEntity {
 
   /// Whether this is a factory constructor, possibly redirecting.
   bool get isFactoryConstructor;
+
+  /// Whether this is a `fromEnvironment` const constructor in `int`, `bool` or
+  /// `String`.
+  bool get isFromEnvironmentConstructor;
 }
 
 /// An entity that defines a local entity (memory slot) in generated code.