Don't crash if a failed dependency is in the root package.

sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart

Index: sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
index a466c2cfc890b2582ea1075a2153e3a6d4152311..3d08e204e3b39fc6e207a5f533127ba68b8a1632 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -298,19 +298,23 @@ class BacktrackingSolver {
     // Generate a reverse dependency graph. For each package, create edges to
     // each package that depends on it.
     var dependers = new Map<String, Set<String>>();
+
+    addDependencies(name, deps) {
+      dependers.putIfAbsent(name, () => new Set<String>());
+      for (var dep in deps) {
+        dependers.putIfAbsent(dep.name, () => new Set<String>()).add(name);
+      }
+    }
+
     for (var i = 0; i < _selected.length; i++) {
       var id = _selected[i].current;
       var pubspec = cache.getCachedPubspec(id);
-      if (pubspec == null) continue;
-      dependers.putIfAbsent(id.name, () => new Set<String>());
-
-      for (var dep in pubspec.dependencies) {
-        var depender = dependers.putIfAbsent(dep.name,
-            () => new Set<String>());
-        depender.add(id.name);
-      }
+      if (pubspec != null) addDependencies(id.name, pubspec.dependencies);
     }
 
+    // Include the root package's dependencies.
+    addDependencies(root.name, root.immediateDependencies);
+
     // Now walk the depending graph to see which packages transitively depend
     // on [dependency].
     var visited = new Set<String>();