dart2js: complain about missing part-of tags.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
@@ skipping 787 matching lines @@
   }
 
   SourceSpan spanFromNode(Node node, [Uri uri]) {
     return spanFromTokens(node.getBeginToken(), node.getEndToken(), uri);
   }
 
   SourceSpan spanFromElement(Element element) {
     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
       // error" or "not implemented yet", so the vicinity is good
       // enough for now.
       element = element.enclosingElement;
       // TODO(ahe): I plan to overhaul this infrastructure anyways.
     }
     if (element == null) {
       element = currentElement;
@@ skipping 121 matching lines @@
   // TODO(johnniwinther): Use [spannable] and [message] to provide better
   // information on assertion errors.
   if (condition is Function){
     condition = condition();
   }
   if (spannable == null || !condition) {
     throw new SpannableAssertionFailure(spannable, message);
   }
   return true;
 }