Enabled reporting problems in some libraries.

compiler/java/com/google/dart/compiler/DartCompiler.java

Index: compiler/java/com/google/dart/compiler/DartCompiler.java
diff --git a/compiler/java/com/google/dart/compiler/DartCompiler.java b/compiler/java/com/google/dart/compiler/DartCompiler.java
index f8dd7d954d2ff61016df55ab0760c96aeaad2ab7..5deca30837677caf68962202055b0d3ce87d6c2c 100644
--- a/compiler/java/com/google/dart/compiler/DartCompiler.java
+++ b/compiler/java/com/google/dart/compiler/DartCompiler.java
@@ -704,11 +704,8 @@ public class DartCompiler {
           // analyze directives
           List<DartDirective> directives = unit.getDirectives();
           if (directives.isEmpty()) {
-            // TODO(scheglov) Remove after http://code.google.com/p/dart/issues/detail?id=6121
-            if (!StringUtils.startsWith(lib.getName(), "dart:")) {
-              context.onError(new DartCompilationError(unitSource,
-                  DartCompilerErrorCode.MISSING_PART_OF_DIRECTIVE, lib.getName()));
-            }
+            context.onError(new DartCompilationError(unitSource,
+                DartCompilerErrorCode.MISSING_PART_OF_DIRECTIVE, lib.getName()));
           } else if (directives.size() == 1 && directives.get(0) instanceof DartPartOfDirective) {
             DartPartOfDirective directive = (DartPartOfDirective) directives.get(0);
             String dirName = directive.getLibraryName();