make dart2js_info strong-mode clean

lib/src/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.
 
 /// A library to work with graphs. It contains a couple algorithms, including
 /// Tarjan's algorithm to compute strongly connected components in a graph and
 /// Cooper et al's dominator algorithm.
 ///
 /// Portions of the code in this library was adapted from
 /// `package:analyzer/src/generated/collection_utilities.dart`.
@@ skipping 25 matching lines @@
       if (identical(node, target)) return true;
       if (!seen.add(node)) return false;
       return targetsOf(node).any(helper);
     }
     return helper(source);
   }
 
   /// Returns all nodes reachable from [root] in post order.
   Iterable<N> postOrder(N root) sync* {
     var seen = new Set<N>();
-    Iterable<N> helper(n) sync* {
+    Iterable<N> helper(N n) sync* {
       if (!seen.add(n)) return;
       for (var x in targetsOf(n)) {
         yield* helper(x);
       }
       yield n;
     }
     yield* helper(root);
   }
 
   /// Returns an iterable of all nodes reachable from [root] in preorder.
@@ skipping 233 matching lines @@
     var nodesInPostOrder = _graph.postOrder(root).toList();
     for (var n in nodesInPostOrder) {
       postOrderId[n] = i++;
     }
     var nodesInReversedPostOrder = nodesInPostOrder.reversed;
     while (changed) {
       changed = false;
       for (var n in nodesInReversedPostOrder) {
         if (n == root) continue;
         bool first = true;
-        var idom = null;
+        N idom;
         for (var p in _graph.sourcesOf(n)) {
           if (immediateDominators[p] != null) {
             if (first) {
               idom = p;
               first = false;
             } else {
               idom = _intersect(p, idom);
             }
           }
         }
@@ skipping 12 matching lines @@
       while (postOrderId[finger1] < postOrderId[finger2]) {
         finger1 = immediateDominators[finger1];
       }
       while (postOrderId[finger2] < postOrderId[finger1]) {
         finger2 = immediateDominators[finger2];
       }
     }
     return finger1;
   }
 }