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..1b2a8707feabbfc7e217ae5511218b6ca3f361fb 100644
--- a/pkg/compiler/lib/src/js_backend/patch_resolver.dart
+++ b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
@@ -34,6 +34,14 @@ class PatchResolverTask extends CompilerTask {
       checkMatchingPatchSignatures(element, patch);
       element = patch;
     } else {
+      if (element.isConstructor) {
+        // Note: currently we allow a couple 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.
+        ConstructorElementX constructor = element;
+        if (constructor.isFromEnvironmentConstructor) return element;
+      }
       reporter.reportErrorMessage(
           element, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
     }