Fix warnings spotted by dart.

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

 // Copyright (c) 2013, 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.
 
 library deferred_load;
 
 import 'dart:uri';
 
 import 'dart2jslib.dart'
        show Compiler,
             CompilerTask,
             ConstructedConstant,
-            SourceString;
+            MessageKind,
+            SourceString,
+            StringConstant;
 
 import 'elements/elements.dart'
        show ClassElement,
             Element,
             LibraryElement,
             MetadataAnnotation;
 
 import 'util/util.dart'
        show Link;
 
@@ skipping 16 matching lines @@
     }
     return cachedDeferredLibraryClass;
   }
 
   ClassElement findDeferredLibraryClass() {
     var uri = new Uri.fromComponents(scheme: 'dart', path: 'async');
     LibraryElement asyncLibrary =
         compiler.libraryLoader.loadLibrary(uri, null, uri);
     var element = asyncLibrary.find(const SourceString('DeferredLibrary'));
     if (element == null) {
-      internalErrorOnElement(
+      compiler.internalErrorOnElement(
           asyncLibrary,
           'dart:async library does not contain required class: '
           'DeferredLibrary');
     }
     return element;
   }
 
   bool isDeferred(Element element) {
     // TODO(ahe): This is really a graph coloring problem. We should
     // make sure that libraries and elements only used by a deferred
     // library are also deferred.
     // Also, if something is deferred depends on your
     // perspective. Inside a deferred library, other elements of the
     // same library are not deferred. We should add an extra parameter
     // to this method to indicate "from where".
     return deferredLibraries.contains(element.getLibrary());
   }
 
   void registerMainApp(LibraryElement mainApp) {
     if (mainApp == null) return;
     measureElement(mainApp, () {
       deferredLibraries.addAll(findDeferredLibraries(mainApp));
     });
   }
 
-  Link<Element> findDeferredLibraries(LibraryElement library) {
+  Link<LibraryElement> findDeferredLibraries(LibraryElement library) {
     Link<LibraryElement> link = const Link<LibraryElement>();
     for (LibraryTag tag in library.tags) {
       Link<MetadataAnnotation> metadata = tag.metadata;
       if (metadata == null) continue;
       for (MetadataAnnotation metadata in tag.metadata) {
         metadata.ensureResolved(compiler);
         Element element = metadata.value.computeType(compiler).element;
         if (element == deferredLibraryClass) {
           ConstructedConstant value = metadata.value;
-          SourceString expectedName = value.fields[0].toDartString().source;
+          StringConstant nameField = value.fields[0];
+          SourceString expectedName = nameField.toDartString().source;
           LibraryElement deferredLibrary = library.getLibraryFromTag(tag);
           link = link.prepend(deferredLibrary);
           SourceString actualName =
               new SourceString(deferredLibrary.getLibraryOrScriptName());
           if (expectedName != actualName) {
             compiler.reportErrorCode(
                 metadata,
                 MessageKind.DEFERRED_LIBRARY_NAME_MISMATCH,
                 { 'expectedName': expectedName.slowToString(),
                   'actualName': actualName.slowToString()});
           }
         }
       }
     }
     return link;
   }
 }