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..03b8c4e984996896b90bafa5036c5bf6ebf2ec11 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -766,12 +766,13 @@ abstract class Compiler implements DiagnosticListener {
     reportDiagnostic(span, "$message", kind);
   }
 
-  void onDeprecatedFeature(Spannable span, String feature) {
+  /// Returns true if a diagnostic was emitted.
+  bool onDeprecatedFeature(Spannable span, String feature) {
     if (currentElement == null)
       throw new SpannableAssertionFailure(span, feature);
     if (!checkDeprecationInSdk &&
         currentElement.getLibrary().isPlatformLibrary) {
-      return;
+      return false;
     }
     var kind = rejectDeprecatedFeatures
         ? api.Diagnostic.ERROR : api.Diagnostic.WARNING;
@@ -779,6 +780,7 @@ abstract class Compiler implements DiagnosticListener {
         ? MessageKind.DEPRECATED_FEATURE_ERROR.error([feature])
         : MessageKind.DEPRECATED_FEATURE_WARNING.error([feature]);
     reportMessage(spanFromSpannable(span), message, kind);
+    return true;
   }
 
   void reportDiagnostic(SourceSpan span, String message, api.Diagnostic kind);
@@ -805,7 +807,7 @@ 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