some fixes for --strong warnings

lib/src/server/dependency_graph.dart

 // Copyright (c) 2015, 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.
 
 /// Tracks the shape of the import/export graph and dependencies between files.
 library dev_compiler.src.dependency_graph;
 
 import 'dart:collection' show HashSet, HashMap;
 
 import 'package:analyzer/analyzer.dart' show parseDirectives;
@@ skipping 184 matching lines @@
       var newScripts = new Set<DartSourceNode>();
       var tags = document.querySelectorAll('script[type="application/dart"]');
       for (var script in tags) {
         var src = script.attributes['src'];
         if (src == null) {
           _reportError(graph, 'inlined script tags not supported at this time '
               '(see https://github.com/dart-lang/dart-dev-compiler/issues/54).',
               script);
           continue;
         }
-        var node = graph.nodeFromUri(uri.resolve(src));
+        DartSourceNode node = graph.nodeFromUri(uri.resolve(src));
         if (node == null || !node.source.exists()) {
           _reportError(graph, 'Script file $src not found', script);
         }
         if (node != null) newScripts.add(node);
       }
 
       if (!_same(newScripts, scripts)) {
         structureChanged = true;
         scripts = newScripts;
       }
 
       // TODO(jmesserly): simplify the design here. Ideally we wouldn't need
       // to track user-defined CSS, images, etc. Also we don't have a clear
       // way to distinguish runtime injected resources, like messages.css, from
       // user-defined files.
       htmlResourceNodes.clear();
       var newResources = new Set<SourceNode>();
       for (var resource in graph.resources) {
         newResources.add(graph.nodeFromUri(uri.resolve(resource)));
       }
       for (var tag in document.querySelectorAll('link[rel="stylesheet"]')) {
-        var res = graph.nodeFromUri(uri.resolve(tag.attributes['href']));
+        ResourceSourceNode res =
+            graph.nodeFromUri(uri.resolve(tag.attributes['href']));

[Leaf, 2015/07/22 22:29:15]

Is there a rationale behind using the T v = e pattern in some places and the var
v = e as T pattern in others?

[Jennifer Messerly, 2015/07/22 22:35:52]

I have no idea ... the messages seemed inconsistent :|
here, I think it complained about htmlResourceNodes requiring
ResourceSourceNode.
Somewhere else, it forced me to switch a `T v = e` to `var e as T` to get rid of
a warning ...

         htmlResourceNodes[tag] = res;
         newResources.add(res);
       }
       for (var tag in document.querySelectorAll('img[src]')) {
-        var res = graph.nodeFromUri(uri.resolve(tag.attributes['src']));
+        ResourceSourceNode res =
+            graph.nodeFromUri(uri.resolve(tag.attributes['src']));
         htmlResourceNodes[tag] = res;
         newResources.add(res);
       }
       if (!_same(newResources, resources)) {
         structureChanged = true;
         resources = newResources;
       }
     }
   }
 
@@ skipping 66 matching lines @@
 
         var directiveUri = (d as UriBasedDirective).uri;
 
         // `dart:core` and other similar URLs only contain a name, but it is
         // meant to be a folder when resolving relative paths from it.
         var targetUri = uri.scheme == 'dart' && uri.pathSegments.length == 1
             ? Uri.parse('$uri/').resolve(directiveUri.stringValue)
             : uri.resolve(directiveUri.stringValue);
         var target =
             ParseDartTask.resolveDirective(graph._context, _source, d, null);
-        var node = graph.nodes.putIfAbsent(
+        DartSourceNode node = graph.nodes.putIfAbsent(
             targetUri, () => new DartSourceNode(graph, targetUri, target));
         //var node = graph.nodeFromUri(targetUri);
         if (node._source == null || !node._source.exists()) {
           _reportError(graph, 'File $targetUri not found', d);
         }
 
         if (d is ImportDirective) {
           newImports.add(node);
         } else if (d is ExportDirective) {
           newExports.add(node);
@@ skipping 170 matching lines @@
     var deps = includeParts ? node.allDeps : node.depsWithoutParts;
     deps.forEach(helper);
     action(node);
   }
   helper(start);
 }
 
 bool _same(Set a, Set b) => a.length == b.length && a.containsAll(b);
 
 final _log = new Logger('dev_compiler.dependency_graph');