Properly process dart:core classes if dart:core is imported with prefix.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index e9af56a075b73bd0ec0d47a1505afc6f1a297342..df713ed45acf83b9befcaa3329ae2c2aef3a446f 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -227,9 +227,12 @@ class DartBackend extends Backend {
             // TODO(antonm): better way to analyze the name.
             fixedMemberNames.add(name.split(@'$').last());
           }
-        } else {
-          fixedMemberNames.add(element.name.slowToString());
         }
+        // Even class names are added due to a delicate problem we have:
+        // if one imports dart:core with a prefix, we cannot tell prefix.name
+        // from dynamic invocation (alas!).  So we'd better err on preserving
+        // those names.
+        fixedMemberNames.add(element.name.slowToString());
       }
     }
     // TODO(antonm): TypeError.srcType and TypeError.dstType are defined in