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

tests/compiler/dart2js_extra/new_from_env_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:expect/expect.dart';
+
+/// Dart2js only supports the const constructor for `?.fromEnvironment`. The
+/// current behavior is to throw at runtime if they call this constructor with
+/// `new` instead of `const`.
+main() {
+  Expect.isFalse(const bool.fromEnvironment('X'));
+  Expect.isNull(const String.fromEnvironment('X'));
+  Expect.equals(const int.fromEnvironment('X', defaultValue: 0), 0);
+
+  Expect.throws(() => new bool.fromEnvironment('X'));
+  Expect.throws(() => new String.fromEnvironment('X'));
+  Expect.throws(() => new int.fromEnvironment('X', defaultValue: 0));
+}