Use engine's getWordBoundary method when user selects a word. (#3851)

packages/flutter/lib/src/painting/text_painter.dart

@@ -229,4 +229,9 @@ class TextPainter {
     return _paragraph.getPositionForOffset(offset);
   }
 
+  TextRange getWordBoundary(TextPosition position) {
+    assert(!_needsLayout);
+    List<int> indices = _paragraph.getWordBoundary(position.offset);
+    return new TextRange(start: indices[0], end: indices[1]);
+  }
 }

packages/flutter/lib/src/rendering/editable_line.dart

@@ -232,19 +232,8 @@ class RenderEditableLine extends RenderBox {
   }
 
   TextSelection _selectWordAtOffset(TextPosition position) {
-    // TODO(mpcomplete): Placeholder. Need to ask the engine for this info to do
-    // it correctly.
-    String str = text.toPlainText();
-    int start = position.offset - 1;
-    while (start >= 0 && str[start] != ' ')
-      --start;
-    ++start;
-
-    int end = position.offset;
-    while (end < str.length && str[end] != ' ')
-      ++end;
-
-    return new TextSelection(baseOffset: start, extentOffset: end);
+    TextRange word = _textPainter.getWordBoundary(position);
+    return new TextSelection(baseOffset: word.start, extentOffset: word.end);
   }
 
   Rect _caretPrototype;