Make dart2js not consider dart:io there if the library is unsupported.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 3bddb9b64661d9ba16578556a1c10f8843561b83..dbff8dfefb0bb81bb8d3804c509280c33506afcd 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -381,7 +381,9 @@ class _Environment implements Environment {
     // Private libraries are not exposed to the users.
     if (libraryName.startsWith("_")) return null;
 
-    if (compiler.resolvedUriTranslator.sdkLibraries.containsKey(libraryName)) {
+    Uri libraryUri =
+        compiler.resolvedUriTranslator.sdkLibraries[libraryName];
+    if (libraryUri != null && libraryUri.scheme != "unsupported") {
       // Dart2js always "supports" importing 'dart:mirrors' but will abort
       // the compilation at a later point if the backend doesn't support
       // mirrors. In this case 'mirrors' should not be in the environment.