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

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

Index: pkg/compiler/lib/src/js_backend/patch_resolver.dart
diff --git a/pkg/compiler/lib/src/js_backend/patch_resolver.dart b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
index 0dfd302191f10ccbfc9ba88b22a7cc28b9adf2f8..79787e1a42d8f8a1e36947006b84fdba7dd7615c 100644
--- a/pkg/compiler/lib/src/js_backend/patch_resolver.dart
+++ b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
@@ -33,7 +33,12 @@ class PatchResolverTask extends CompilerTask {
       });
       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);
     }