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 0f9ec2b54a12308d9798526044d4b42cdb481b2d..128df108c95ddf27173dac1ee374a41cd8628fa2 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -212,9 +212,12 @@ 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