AAPT2: Allow undefined resources (placeholders)

A resource defined like so:

<item type="drawable" name="foo" />

should be assigned the value @null.

The only exception is for <string> resources, which are given the
empty string value (since <string></string> is ambiguous). The decision
to use "" is based off the fact that old AAPT used to assign "" to all
undefined resources, even non-string ones.

Bug: 38425050
Test: make aapt2_tests
Change-Id: Ib3e0f6f83d16ddd8b279c9fd44a07a37867b85e9
diff --git a/tools/aapt2/test/Common.cpp b/tools/aapt2/test/Common.cpp
new file mode 100644
index 0000000..0fabbc4
--- /dev/null
+++ b/tools/aapt2/test/Common.cpp
@@ -0,0 +1,60 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "test/Common.h"
+
+namespace aapt {
+namespace test {
+
+struct DummyDiagnosticsImpl : public IDiagnostics {
+  void Log(Level level, DiagMessageActual& actual_msg) override {
+    switch (level) {
+      case Level::Note:
+        return;
+
+      case Level::Warn:
+        std::cerr << actual_msg.source << ": warn: " << actual_msg.message << "." << std::endl;
+        break;
+
+      case Level::Error:
+        std::cerr << actual_msg.source << ": error: " << actual_msg.message << "." << std::endl;
+        break;
+    }
+  }
+};
+
+IDiagnostics* GetDiagnostics() {
+  static DummyDiagnosticsImpl diag;
+  return &diag;
+}
+
+template <>
+Value* GetValueForConfigAndProduct<Value>(ResourceTable* table,
+                                          const android::StringPiece& res_name,
+                                          const ConfigDescription& config,
+                                          const android::StringPiece& product) {
+  Maybe<ResourceTable::SearchResult> result = table->FindResource(ParseNameOrDie(res_name));
+  if (result) {
+    ResourceConfigValue* config_value = result.value().entry->FindValue(config, product);
+    if (config_value) {
+      return config_value->value.get();
+    }
+  }
+  return nullptr;
+}
+
+}  // namespace test
+}  // namespace aapt