Don't have polymer thinking unmodifiable_collection is in pkg.

pkg/polymer/lib/src/build/runner.dart

Index: pkg/polymer/lib/src/build/runner.dart
diff --git a/pkg/polymer/lib/src/build/runner.dart b/pkg/polymer/lib/src/build/runner.dart
index af71282fab953b236b61390174a8700483f1a950..c5f13d2f760e5a1e78c3a332bd32a5d1c424b808 100644
--- a/pkg/polymer/lib/src/build/runner.dart
+++ b/pkg/polymer/lib/src/build/runner.dart
@@ -117,11 +117,10 @@ Map<String, String> _readPackageDirsFromPub(String currentPackage) {
 // TODO(sigmund): consider computing this list by recursively parsing
 // pubspec.yaml files in the `Options.packageDirs`.
 final Set<String> _polymerPackageDependencies = [
-    'analyzer', 'args', 'barback', 'browser', 'custom_element', 'html5lib',
-    'html_import', 'js', 'logging', 'mutation_observer', 'observe', 'path'
-    'polymer_expressions', 'serialization', 'shadow_dom', 'source_maps',
-    'stack_trace', 'template_binding', 'unittest', 'unmodifiable_collection',
-    'yaml'].toSet();
+    'analyzer', 'args', 'barback', 'browser', 'collection', 'custom_element',
+    'html5lib','html_import', 'js', 'logging', 'mutation_observer', 'observe',
+    'path', 'polymer_expressions', 'serialization', 'shadow_dom', 'source_maps',
+    'stack_trace', 'template_binding', 'unittest', 'yaml'].toSet();
 
 /** Return the relative path of each file under [subDir] in [package]. */
 Iterable<String> _listPackageDir(String package, String subDir,