Add support for RCTText under FlatUIImplementation

Summary: @public Initial version of FlatUIImplementation lacks any primitives support (such as RCTText, RCTImageView or RCTView). This diff add the first part, RCTText (alongside with RCTVirtualText and RCTRawText).

Reviewed By: sriramramani

Differential Revision: D2693348

ReactAndroid/src/main/java/com/facebook/react/flat/FlatTextShadowNode.java

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+package com.facebook.react.flat;
+
+import android.text.SpannableStringBuilder;
+
+import com.facebook.react.uimanager.ReactShadowNode;
+
+/**
+ * Base class for RCTVirtualText and RCTRawText.
+ */
+/* package */ abstract class FlatTextShadowNode extends FlatShadowNode {
+
+  /**
+   * Recursively visits FlatTextShadowNode and its children,
+   * appending text to SpannableStringBuilder.
+   */
+  protected abstract void collectText(SpannableStringBuilder builder);
+
+  /**
+   * Recursively visits FlatTextShadowNode and its children,
+   * applying spans to SpannableStringBuilder.
+   */
+  protected abstract void applySpans(SpannableStringBuilder builder);
+
+  /**
+   * Propagates changes up to RCTText without dirtying current node.
+   */
+  protected void notifyChanged(boolean shouldRemeasure) {
+    ReactShadowNode parent = getParent();
+    if (parent instanceof FlatTextShadowNode) {
+      ((FlatTextShadowNode) parent).notifyChanged(shouldRemeasure);
+    }
+  }
+
+  @Override
+  public boolean isVirtual() {
+    return true;
+  }
+}