Don't expose private libraries with dart.library.X.

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 e84777f59200c13be55ae623671dc2bbc1e23be2..73bbc60d3f11156bb00eab50ce608adc1c7257f6 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -597,11 +597,15 @@ class CompilerImpl extends Compiler {
     if (!name.startsWith(dartLibraryEnvironmentPrefix)) return null;
 
     String libraryName = name.substring(dartLibraryEnvironmentPrefix.length);
+
+    // Private libraries are not exposed to the users.
+    if (libraryName.startsWith("_")) return null;
+
     if (sdkLibraries.containsKey(libraryName)) {
       // 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.
-      if (name == dartLibraryEnvironmentPrefix + 'mirrors') {
+      if (libraryName == 'mirrors') {
         return backend.supportsReflection ? "true" : null;
       }
       return "true";