Validate UTF-16 input in ParagraphBuilder::addText (#4300)

Fixes https://github.com/flutter/flutter/issues/12772

lib/ui/text.dart

@@ -970,7 +970,12 @@ class ParagraphBuilder extends NativeFieldWrapperClass2 {
   /// Adds the given text to the paragraph.
   ///
   /// The text will be styled according to the current stack of text styles.
-  void addText(String text) native "ParagraphBuilder_addText";
+  void addText(String text) {
+    String error = _addText(text);
+    if (error != null)
+      throw new ArgumentError(error);
+  }
+  String _addText(String text) native "ParagraphBuilder_addText";
 
   /// Applies the given paragraph style and returns a [Paragraph] containing the
   /// added text and associated styling.

lib/ui/text/paragraph_builder.cc

@@ -417,19 +417,34 @@ void ParagraphBuilder::pop() {
   }
 }
 
-void ParagraphBuilder::addText(const std::string& text) {
+Dart_Handle ParagraphBuilder::addText(const std::u16string& text) {
+  if (text.empty())
+    return Dart_Null();
+
+  // Use ICU to validate the UTF-16 input.  Calling u_strToUTF8 with a null
+  // output buffer will return U_BUFFER_OVERFLOW_ERROR if the input is well
+  // formed.
+  const UChar* text_ptr = reinterpret_cast<const UChar*>(text.data());
+  UErrorCode error_code = U_ZERO_ERROR;
+  u_strToUTF8(nullptr, 0, nullptr, text_ptr, text.size(), &error_code);
+  if (error_code != U_BUFFER_OVERFLOW_ERROR)
+    return tonic::ToDart("string is not well-formed UTF-16");
+
   if (!Settings::Get().using_blink) {
     m_paragraphBuilder->AddText(text);
   } else {
     // Blink Version.
     if (!m_currentRenderObject)
-      return;
-    RenderText* renderText = new RenderText(String::fromUTF8(text).impl());
+      return tonic::ToDart("paragraph has already been built");
+    RenderText* renderText =
+        new RenderText(String(text_ptr, text.size()).impl());
     RefPtr<RenderStyle> style = RenderStyle::create();
     style->inheritFrom(m_currentRenderObject->style());
     renderText->setStyle(style.release());
     m_currentRenderObject->addChild(renderText);
   }
+
+  return Dart_Null();
 }
 
 fxl::RefPtr<Paragraph> ParagraphBuilder::build() {

lib/ui/text/paragraph_builder.h

@@ -44,7 +44,7 @@ class ParagraphBuilder : public fxl::RefCountedThreadSafe<ParagraphBuilder>,
 
   void pop();
 
-  void addText(const std::string& text);
+  Dart_Handle addText(const std::u16string& text);
 
   fxl::RefPtr<Paragraph> build();