Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/test/lock_file_test.dart

Index: sdk/lib/_internal/pub/test/lock_file_test.dart
diff --git a/sdk/lib/_internal/pub/test/lock_file_test.dart b/sdk/lib/_internal/pub/test/lock_file_test.dart
index 0f7fe53875149a9ec29380ea88bb1e654c9ee19b..9816b86452b10cf78f73559ed79c1d74103adc14 100644
--- a/sdk/lib/_internal/pub/test/lock_file_test.dart
+++ b/sdk/lib/_internal/pub/test/lock_file_test.dart
@@ -65,13 +65,13 @@ packages:
         var bar = lockFile.packages['bar'];
         expect(bar.name, equals('bar'));
         expect(bar.version, equals(new Version(1, 2, 3)));
-        expect(bar.source, equals(mockSource));
+        expect(bar.source, equals(mockSource.name));
         expect(bar.description, equals('bar desc'));
 
         var foo = lockFile.packages['foo'];
         expect(foo.name, equals('foo'));
         expect(foo.version, equals(new Version(2, 3, 4)));
-        expect(foo.source, equals(mockSource));
+        expect(foo.source, equals(mockSource.name));
         expect(foo.description, equals('foo desc'));
       });
 
@@ -166,9 +166,9 @@ packages:
 
       test('dumps the lockfile to YAML', () {
         lockfile.packages['foo'] = new PackageId(
-            'foo', mockSource, new Version.parse('1.2.3'), 'foo desc');
+            'foo', mockSource.name, new Version.parse('1.2.3'), 'foo desc');
         lockfile.packages['bar'] = new PackageId(
-            'bar', mockSource, new Version.parse('3.2.1'), 'bar desc');
+            'bar', mockSource.name, new Version.parse('3.2.1'), 'bar desc');
 
         expect(loadYaml(lockfile.serialize()), equals({
           'packages': {
@@ -189,7 +189,7 @@ packages:
       test('lockfile is alphabetized by package name', () {
         var testNames = ['baz', 'Qwe', 'Q', 'B', 'Bar', 'bar', 'foo'];
         testNames.forEach((name) {
-          lockfile.packages[name] = new PackageId(name, mockSource,
+          lockfile.packages[name] = new PackageId(name, mockSource.name,
             new Version.parse('5.5.5'), '$name desc');
         });