Track Source objects in PackageNames.

lib/src/solver/backtracking_solver.dart

Index: lib/src/solver/backtracking_solver.dart
diff --git a/lib/src/solver/backtracking_solver.dart b/lib/src/solver/backtracking_solver.dart
index e36ff079d62d2ce513b50abe62f90dc1239a25f3..7f3cc432fe51ca74cdfcccc14cfb47c0335aa02c 100644
--- a/lib/src/solver/backtracking_solver.dart
+++ b/lib/src/solver/backtracking_solver.dart
@@ -231,10 +231,7 @@ class BacktrackingSolver {
     // can't be downgraded.
     if (type == SolveType.DOWNGRADE) {
       var locked = lockFile.packages[package];
-      if (locked != null &&
-          !systemCache.sources[locked.source].hasMultipleVersions) {
-        return locked;
-      }
+      if (locked != null && !locked.source.hasMultipleVersions) return locked;
     }
 
     if (_forceLatest.isEmpty || _forceLatest.contains(package)) return null;
@@ -259,13 +256,7 @@ class BacktrackingSolver {
     }
 
     var required = _selection.getRequiredDependency(name);
-    if (required != null) {
-      if (package.source != required.dep.source) return null;
-
-      var source = systemCache.sources[package.source];
-      if (!source.descriptionsEqual(
-          package.description, required.dep.description)) return null;
-    }
+    if (required != null && !package.samePackage(required.dep)) return null;
 
     return package;
   }
@@ -536,9 +527,7 @@ class BacktrackingSolver {
         throw new SourceMismatchException(dep.name, allDeps);
       }
 
-      var source = systemCache.sources[dep.source];
-      if (!source.descriptionsEqual(
-          dep.description, required.dep.description)) {
+      if (!dep.samePackage(required.dep)) {
         // Mark the dependers as failing rather than the package itself, because
         // no version with this description will be compatible.
         for (var otherDep in _selection.getDependenciesOn(dep.name)) {
@@ -595,7 +584,7 @@ class BacktrackingSolver {
 
     // Make sure the package doesn't have any bad dependencies.
     for (var dep in deps.toSet()) {
-      if (!dep.isRoot && systemCache.sources[dep.source] is UnknownSource) {
+      if (!dep.isRoot && dep.source is UnknownSource) {
         throw new UnknownSourceException(id.name, [new Dependency(id, dep)]);
       }