dart2js: complain about missing part-of tags.

dart/sdk/lib/_internal/compiler/implementation/compiler.dart

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index 99cfb248bfac316fb805738aad4873d7fedb17f1..2bcd622ae19e9f6c85abc026ffe58aa11ce810bd 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -805,7 +805,7 @@ abstract class Compiler implements DiagnosticListener {
     if (Elements.isErroneousElement(element)) {
       element = element.enclosingElement;
     }
-    if (element.position() == null) {
+    if (element.position() == null && !element.isCompilationUnit()) {
       // Sometimes, the backend fakes up elements that have no
       // position. So we use the enclosing element instead. It is
       // not a good error location, but cancel really is "internal