Access to imports in the VM's runtime mirrors. Extend test coverage of the source mirrors.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index a3700d602252b85f548d336bd304b6a772f2fd72..d150745e95d60ffc2d2552c75c42ad9316f58c4e 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -806,8 +806,10 @@ class RawNamespace : public RawObject {
   RawLibrary* library_;          // library with name dictionary.
   RawArray* show_names_;         // list of names that are exported.
   RawArray* hide_names_;         // blacklist of names that are not exported.
+  RawField* metadata_field_;     // remembers the token pos of metadata if any,
+                                 // and the metadata values if computed.
   RawObject** to() {
-    return reinterpret_cast<RawObject**>(&ptr()->hide_names_);
+    return reinterpret_cast<RawObject**>(&ptr()->metadata_field_);
   }
 };