Deal better with missing sources in code_transformers and polymer

pkg/code_transformers/lib/src/resolver_impl.dart

Index: pkg/code_transformers/lib/src/resolver_impl.dart
diff --git a/pkg/code_transformers/lib/src/resolver_impl.dart b/pkg/code_transformers/lib/src/resolver_impl.dart
index ad73da401ae44cc2fc5e5ee674626b26234b44e1..10e323c029b84707bfc3e2e72e2bc2094afadc2a 100644
--- a/pkg/code_transformers/lib/src/resolver_impl.dart
+++ b/pkg/code_transformers/lib/src/resolver_impl.dart
@@ -159,8 +159,11 @@ class ResolverImpl implements Resolver {
       _context.applyChanges(changeSet);
       // Force resolve each entry point (the getter will ensure the library is
       // computed first).
-      _entryLibraries = entryPoints
-          .map((id) => _context.computeLibraryElement(sources[id])).toList();
+      _entryLibraries = entryPoints.map((id) {
+        var source = sources[id];
+        if (source == null) return null;
+        return _context.computeLibraryElement(source);
+      }).toList();
     });
   }