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

pkg/compiler/lib/src/js_backend/impact_transformer.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.
 
 library js_backend.backend.impact_transformer;
 
 import '../common.dart';
 import '../common_elements.dart';
 import '../common/backend_api.dart' show ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 121 matching lines @@
           break;
         case Feature.THROW_EXPRESSION:
           registerImpact(_impacts.throwExpression);
           break;
         case Feature.THROW_NO_SUCH_METHOD:
           registerImpact(_impacts.throwNoSuchMethod);
           break;
         case Feature.THROW_RUNTIME_ERROR:
           registerImpact(_impacts.throwRuntimeError);
           break;
+        case Feature.THROW_UNSUPPORTED_ERROR:
+          registerImpact(_impacts.throwUnsupportedError);
+          break;
         case Feature.TYPE_VARIABLE_BOUNDS_CHECK:
           registerImpact(_impacts.typeVariableBoundCheck);
           break;
       }
     }
 
     bool hasAsCast = false;
     bool hasTypeLiteral = false;
     for (TypeUse typeUse in worldImpact.typeUses) {
       DartType type = typeUse.type;
@@ skipping 315 matching lines @@
           _impacts.asyncStarBody
               .registerImpact(transformed, _elementEnvironment);
           break;
       }
     }
 
     // TODO(johnniwinther): Remove eager registration.
     return transformed;
   }
 }