Refactor resolution code in the vm to properly handle ambiguity errors.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 25321)
+++ runtime/vm/object.h	(working copy)
@@ -2286,14 +2286,23 @@
   void AddObject(const Object& obj, const String& name) const;
   void ReplaceObject(const Object& obj, const String& name) const;
   RawObject* LookupExport(const String& name) const;
-  RawObject* LookupObject(const String& name) const;
-  RawClass* LookupClass(const String& name) const;
-  RawClass* LookupClassAllowPrivate(const String& name) const;
+  RawObject* LookupObject(const String& name,
+                          String* ambiguity_error_msg) const;
+  RawObject* LookupObjectAllowPrivate(const String& name,
+                                      String* ambiguity_error_msg) const;
+  RawObject* LookupLocalObjectAllowPrivate(const String& name) const;
   RawObject* LookupLocalObject(const String& name) const;
+  RawObject* LookupImportedObject(const String& name,
+                                  String* ambiguity_error_msg) const;
+  RawClass* LookupClass(const String& name, String* ambiguity_error_msg) const;
+  RawClass* LookupClassAllowPrivate(const String& name,
+                                    String* ambiguity_error_msg) const;
   RawClass* LookupLocalClass(const String& name) const;
-  RawField* LookupFieldAllowPrivate(const String& name) const;
+  RawField* LookupFieldAllowPrivate(const String& name,
+                                    String* ambiguity_error_msg) const;
   RawField* LookupLocalField(const String& name) const;
-  RawFunction* LookupFunctionAllowPrivate(const String& name) const;
+  RawFunction* LookupFunctionAllowPrivate(const String& name,
+                                          String* ambiguity_error_msg) const;
   RawFunction* LookupLocalFunction(const String& name) const;
   RawLibraryPrefix* LookupLocalLibraryPrefix(const String& name) const;
   RawScript* LookupScript(const String& url) const;
@@ -2427,7 +2436,8 @@
   bool ContainsLibrary(const Library& library) const;
   RawLibrary* GetLibrary(int index) const;
   void AddImport(const Namespace& import) const;
-  RawClass* LookupLocalClass(const String& class_name) const;
+  RawClass* LookupClass(const String& class_name,
+                        String* ambiguity_error_msg) const;
 
   static intptr_t InstanceSize() {
     return RoundedAllocationSize(sizeof(RawLibraryPrefix));