Correctly compile unresolved for-in loops.

dart/tests/compiler/dart2js/mock_compiler.dart

Index: dart/tests/compiler/dart2js/mock_compiler.dart
diff --git a/dart/tests/compiler/dart2js/mock_compiler.dart b/dart/tests/compiler/dart2js/mock_compiler.dart
index 585397ebdab12bcf65308947dcfb0458e6d21b04..97e797d4c94f9a89cabc81deab0dc39293b59fcb 100644
--- a/dart/tests/compiler/dart2js/mock_compiler.dart
+++ b/dart/tests/compiler/dart2js/mock_compiler.dart
@@ -236,7 +236,8 @@ class MockCompiler extends Compiler {
   }
 
   void reportWarning(Node node, var message) {
-    warnings.add(new WarningMessage(node, message.message));
+    if (message is! Message) message = message.message;
+    warnings.add(new WarningMessage(node, message));
   }
 
   void reportError(Node node, var message) {
@@ -244,7 +245,8 @@ class MockCompiler extends Compiler {
       // TODO(ahe): Fix the MockCompiler to not have this problem.
       return;
     }
-    errors.add(new WarningMessage(node, message.message));
+    if (message is! Message) message = message.message;
+    errors.add(new WarningMessage(node, message));
   }
 
   void reportMessage(SourceSpan span, var message, api.Diagnostic kind) {