Gracefully handle pubspecs with dependencies using unknown sources.

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 97d35dad9e930cd0bc787dbe2762d4de414e7653..e2fdc48afec00ebb0f82addeeabbe3e35117e9a1 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -387,6 +387,14 @@ class Traverser {
       // Include dev dependencies of the root package.
       if (id.isRoot) deps.addAll(pubspec.devDependencies);
 
+      // Make sure the package doesn't have any bad dependencies.
+      for (var dep in deps) {
+        if (!dep.isRoot && !_solver.sources.contains(dep.source)) {
+          throw new UnknownSourceException(id.name,
+              [new Dependency(id.name, dep)]);
+        }
+      }
+
       // Given a package dep, returns a future that completes to a pair of the
       // dep and the number of versions available for it.
       getNumVersions(PackageDep dep) {
@@ -395,7 +403,7 @@ class Traverser {
           return new Future.value(new Pair<PackageDep, int>(dep, 1));
         }
 
-        return _solver.cache.getVersions(dep.toRef()).then((versions) {
+        return _solver.cache.getVersions(dep).then((versions) {
           return new Pair<PackageDep, int>(dep, versions.length);
         }).catchError((error) {
           // If it fails for any reason, just treat that as no versions. This
@@ -474,7 +482,7 @@ class Traverser {
     if (required == null) return;
 
     // Make sure all of the existing sources match the new reference.
-    if (required.dep.source.name != dep.source.name) {
+    if (required.dep.source != dep.source) {
       _solver.logSolve('source mismatch on ${dep.name}: ${required.dep.source} '
                        '!= ${dep.source}');
       throw new SourceMismatchException(dep.name,
@@ -482,7 +490,8 @@ class Traverser {
     }
 
     // Make sure all of the existing descriptions match the new reference.
-    if (!dep.descriptionEquals(required.dep)) {
+    var source = _solver.sources[dep.source];
+    if (!source.descriptionsEqual(dep.description, required.dep.description)) {
       _solver.logSolve('description mismatch on ${dep.name}: '
                        '${required.dep.description} != ${dep.description}');
       throw new DescriptionMismatchException(dep.name,
@@ -542,7 +551,7 @@ class Traverser {
   /// could not be selected or that completes successfully if a package was
   /// selected and traversing should continue.
   Future _selectPackage(PackageDep dep, VersionConstraint constraint) {
-    return _solver.cache.getVersions(dep.toRef()).then((versions) {
+    return _solver.cache.getVersions(dep).then((versions) {
       var allowed = versions.where((id) => constraint.allows(id.version));
 
       // See if it's in the lockfile. If so, try that version first. If the
@@ -608,8 +617,11 @@ class Traverser {
 
     var required = _getRequired(name);
     if (required != null) {
-      if (package.source.name != required.dep.source.name) return null;
-      if (!package.descriptionEquals(required.dep)) return null;
+      if (package.source != required.dep.source) return null;
+
+      var source = _solver.sources[package.source];
+      if (!source.descriptionsEqual(
+          package.description, required.dep.description)) return null;
     }
 
     return package;