Ignore 'dart-ext:' scheme, allow 'native' in such files

compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java

Index: compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java
diff --git a/compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java b/compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java
index 4d7aa25052ce004b1c44e62c1ea17d923fa7a6f7..a2fd2c447f8d47acb46b9b5857c1d61a6fe1f4d0 100644
--- a/compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java
+++ b/compiler/javatests/com/google/dart/compiler/end2end/inc/MemoryLibrarySource.java
@@ -9,6 +9,7 @@ import com.google.dart.compiler.LibrarySource;
 import com.google.dart.compiler.Source;
 import com.google.dart.compiler.UrlDartSource;
 
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.Reader;
 import java.io.StringReader;
@@ -67,6 +68,9 @@ public class MemoryLibrarySource implements LibrarySource {
     if (IO_EXCEPTION_CONTENT.equals(content)) {
       throw new IOException("simulated");
     }
+    if (content == null) {
+      throw new FileNotFoundException(libName);
+    }
     return new StringReader(content);
   }