Fix some warnings in the core library identified by the static analyzer.

runtime/lib/string_buffer.dart

Index: runtime/lib/string_buffer.dart
diff --git a/runtime/lib/string_buffer.dart b/runtime/lib/string_buffer.dart
index 4b276d8b6e4161b83872c0fc8c9be0c5847174be..1e7dba0b73bf6e74112d6934cf412cc31e210e81 100644
--- a/runtime/lib/string_buffer.dart
+++ b/runtime/lib/string_buffer.dart
@@ -43,7 +43,7 @@ class StringBufferImpl implements StringBuffer {
   /**
    * Adds all items in [objects] to the buffer. Returns [this].
    */
-  StringBuffer addAll(Collection<Object> objects) {
+  StringBuffer addAll(Collection objects) {
     for (Object obj in objects) {
       add(obj);
     }