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

runtime/vm/mirrors_api_impl.cc

Index: runtime/vm/mirrors_api_impl.cc
===================================================================
--- runtime/vm/mirrors_api_impl.cc	(revision 25321)
+++ runtime/vm/mirrors_api_impl.cc	(working copy)
@@ -342,22 +342,27 @@
     const Library& lib = Library::Cast(obj);
 
     // Case 1.  Lookup the unmodified function name.
-    func = lib.LookupFunctionAllowPrivate(func_name);
+    String& ambiguity_error_msg = String::Handle(isolate);
+    func = lib.LookupFunctionAllowPrivate(func_name, &ambiguity_error_msg);
 
     // Case 2.  Lookup the function without the external setter suffix
     // '='.  Make sure to do this check after the regular lookup, so
     // that we don't interfere with operator lookups (like ==).
-    if (func.IsNull() && HasExternalSetterSuffix(func_name)) {
+    if (func.IsNull() && ambiguity_error_msg.IsNull() &&
+        HasExternalSetterSuffix(func_name)) {
       tmp_name = RemoveExternalSetterSuffix(func_name);
       tmp_name = Field::SetterName(tmp_name);
-      func = lib.LookupFunctionAllowPrivate(tmp_name);
+      func = lib.LookupFunctionAllowPrivate(tmp_name, &ambiguity_error_msg);
     }
 
     // Case 3.  Lookup the function with the getter prefix prepended.
-    if (func.IsNull()) {
+    if (func.IsNull() && ambiguity_error_msg.IsNull()) {
       tmp_name = Field::GetterName(func_name);
-      func = lib.LookupFunctionAllowPrivate(tmp_name);
+      func = lib.LookupFunctionAllowPrivate(tmp_name, &ambiguity_error_msg);
     }
+    if (!ambiguity_error_msg.IsNull()) {
+      return Api::NewError("%s.", ambiguity_error_msg.ToCString());
+    }
   } else {
     return Api::NewError(
         "%s expects argument 'target' to be a class or library.",
@@ -664,7 +669,13 @@
   }
   if (obj.IsLibrary()) {
     const Library& lib = Library::Cast(obj);
-    return Api::NewHandle(isolate, lib.LookupFieldAllowPrivate(var_name));
+    String& ambiguity_error_msg = String::Handle(isolate);
+    const Field& variable = Field::Handle(
+        lib.LookupFieldAllowPrivate(var_name, &ambiguity_error_msg));
+    if (!ambiguity_error_msg.IsNull()) {
+      return Api::NewError("%s.", ambiguity_error_msg.ToCString());
+    }
+    return Api::NewHandle(isolate, variable.raw());
   }
   return Api::NewError(
       "%s expects argument 'target' to be a class or library.",