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

BUG=https://code.google.com/p/dart/issues/detail?id=15550
R=nweiz@google.com

Review URL: https://codereview.chromium.org//112473002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@31071 260f80e4-7a28-3924-810f-c04153c831b5

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>();

test/version_solver_test.dart

@@ -402,6 +402,16 @@ unsolvable() {
       'a': '1.0.0'
     }
   }, error: couldNotSolve, maxTries: 4);
+
+  // This is a regression test for #15550.
+  testResolve('no version that matches while backtracking', {
+    'myapp 0.0.0': {
+      'a': 'any',
+      'b': '>1.0.0'
+    },
+    'a 1.0.0': {},
+    'b 1.0.0': {}
+  }, error: noVersion(['myapp']), maxTries: 1);
 }
 
 badSource() {