Don't crash when a file is missing.

dart/lib/compiler/implementation/apiimpl.dart

Index: dart/lib/compiler/implementation/apiimpl.dart
diff --git a/dart/lib/compiler/implementation/apiimpl.dart b/dart/lib/compiler/implementation/apiimpl.dart
index f983f5a86588f77b8185b0fd7ed0498d272e0e1f..709f190b4bea4c7494f4a8bd55b2d671d9b214de 100644
--- a/dart/lib/compiler/implementation/apiimpl.dart
+++ b/dart/lib/compiler/implementation/apiimpl.dart
@@ -54,8 +54,7 @@ class Compiler extends leg.Compiler {
       if (node !== null) {
         cancel("$exception", node: node);
       } else {
-        reportDiagnostic(const leg.SourceSpan(null, null, null),
-                         "$exception", api.Diagnostic.ERROR);
+        reportDiagnostic(null, "$exception", api.Diagnostic.ERROR);
         throw new leg.CompilerCancelledException("$exception");
       }
     }