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

pkg/compiler/lib/src/js_backend/backend_impact.dart

 // Copyright (c) 2015, 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 dart2js.js_helpers.impact;
 
 import '../common/names.dart';
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
 import '../elements/types.dart' show InterfaceType;
 import '../elements/entities.dart';
@@ skipping 285 matching lines @@
               ]
             : [
                 _commonElements.throwRuntimeError,
               ],
         otherImpacts: [
           // Also register the types of the arguments passed to this method.
           stringValues
         ]);
   }
 
+  BackendImpact _throwUnsupportedError;
+
+  BackendImpact get throwUnsupportedError {
+    return _throwUnsupportedError ??= new BackendImpact(staticUses: [
+      _commonElements.throwUnsupportedError
+    ], otherImpacts: [
+      // Also register the types of the arguments passed to this method.
+      stringValues
+    ]);
+  }
+
   BackendImpact _superNoSuchMethod;
 
   BackendImpact get superNoSuchMethod {
     return _superNoSuchMethod ??= new BackendImpact(staticUses: [
       _commonElements.createInvocationMirror,
       _commonElements.objectNoSuchMethod
     ], otherImpacts: [
       _needsInt('Needed to encode the invocation kind of super.noSuchMethod.'),
       _needsList('Needed to encode the arguments of super.noSuchMethod.'),
       _needsString('Needed to encode the name of super.noSuchMethod.')
@@ skipping 476 matching lines @@
   /// Backend impact for type variables through mirrors.
   BackendImpact get typeVariableMirror {
     return _typeVariableMirror ??= new BackendImpact(staticUses: [
       _commonElements.typeVariableConstructor,
       _commonElements.createRuntimeType
     ], instantiatedClasses: [
       _commonElements.typeVariableClass
     ]);
   }
 }