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

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

 // Copyright (c) 2014, 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_backend.patch_resolver;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
@@ skipping 15 matching lines @@
   String get name => 'JavaScript patch resolver';
 
   FunctionElement resolveExternalFunction(FunctionElementX element) {
     if (element.isPatched) {
       FunctionElementX patch = element.patch;
       reporter.withCurrentElement(patch, () {
         patch.computeType(resolution);
       });
       checkMatchingPatchSignatures(element, patch);
       element = patch;
-    } else {
+    } else if (!element.isConstructor ||
+        !element.isFromEnvironmentConstructor) {
+      // Note: currently we allow a handful of external methods without a patch,
+      // namely the *.fromEnvironment const constructors in int, bool, and
+      // String.  In the future we might also represent native DOM methods in
+      // dart:html this way.
       reporter.reportErrorMessage(
           element, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
     }
     return element;
   }
 
   void checkMatchingPatchParameters(FunctionElement origin,
       List<Element> originParameters, List<Element> patchParameters) {
     bool isUnnamedListConstructor = origin is ConstructorElement &&
         compiler.commonElements.isUnnamedListConstructor(origin);
@@ skipping 129 matching lines @@
           'originParameterCount': originSignature.optionalParameterCount,
           'patchParameterCount': patchSignature.optionalParameterCount
         });
       });
     } else {
       checkMatchingPatchParameters(origin, originSignature.optionalParameters,
           patchSignature.optionalParameters);
     }
   }
 }