Fix path bug in windows, don't emit code altogether if we have already seen

lib/src/compiler.dart

Index: lib/src/compiler.dart
diff --git a/lib/src/compiler.dart b/lib/src/compiler.dart
index 82b2832f00624fa318d0b93b59a16f306b21ee48..c4c15ba877e0aa0ecb7fdf29433f5e0ccfb684d0 100644
--- a/lib/src/compiler.dart
+++ b/lib/src/compiler.dart
@@ -107,11 +107,11 @@ class Compiler {
     }
     return _parseAndDiscover(_mainPath).then((_) {
       _analyze();
-      _transformDart();
-      _emit();
       // TODO(jmesserly): need to go through our errors, and figure out if some
       // of them should be warnings instead.
-      if (_messages.hasErrors) output.clear();
+      if (_messages.hasErrors) return;
+      _transformDart();
+      _emit();
     });
   }