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..716cac42a7cfc24ccb19ec383678a8bf3af853a5 100644
--- a/lib/src/compiler.dart
+++ b/lib/src/compiler.dart
@@ -107,7 +107,9 @@ class Compiler {
     }
     return _parseAndDiscover(_mainPath).then((_) {
       _analyze();
+      if (_messages.hasErrors) return;
       _transformDart();
+      if (_messages.hasErrors) return;

[Jennifer Messerly, 2013/03/19 23:36:04]

this should clear outputs like the other if below. IIRC, _transformDart can emit
files, see _emitModifiedDartFiles.

[Siggi Cherem (dart-lang), 2013/03/19 23:44:53]

Done (see below)

       _emit();
       // TODO(jmesserly): need to go through our errors, and figure out if some
       // of them should be warnings instead.