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

sdk/lib/_internal/js_runtime/lib/js_helper.dart

 // Copyright (c) 2013, 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.
 
 library _js_helper;
 
 import 'dart:_js_embedded_names'
     show
         DEFERRED_LIBRARY_URIS,
         DEFERRED_LIBRARY_HASHES,
@@ skipping 1793 matching lines @@
  * expression context.
  */
 throwExpression(ex) {
   JS('void', 'throw #', wrapException(ex));
 }
 
 throwRuntimeError(message) {
   throw new RuntimeError(message);
 }
 
+throwUnsupportedError(message) {
+  throw new UnsupportedError(message);
+}
+
 throwAbstractClassInstantiationError(className) {
   throw new AbstractClassInstantiationError(className);
 }
 
 // This is used in open coded for-in loops on arrays.
 //
 //     checkConcurrentModificationError(a.length == startLength, a)
 //
 // is replaced in codegen by:
 //
@@ skipping 1977 matching lines @@
 // unneeded code.
 class _UnreachableError extends AssertionError {
   _UnreachableError();
   String toString() => "Assertion failed: Reached dead code";
 }
 
 @NoInline()
 void assertUnreachable() {
   throw new _UnreachableError();
 }