Tweak toString() for better type inference

tools/dom/templates/html/impl/impl_Node.darttemplate

Index: tools/dom/templates/html/impl/impl_Node.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Node.darttemplate b/tools/dom/templates/html/impl/impl_Node.darttemplate
index 830714911d3c7cd90aa1f49e69c050f15c38188b..166c34603b2628f9eec8965cb68625f3c97eb82e 100644
--- a/tools/dom/templates/html/impl/impl_Node.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Node.darttemplate
@@ -272,6 +272,9 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
   /**
    * Print out a String representation of this Node.
    */
-  String toString() => nodeValue == null ? super.toString() : nodeValue;
+  String toString() {
+    String value = nodeValue;  // Fetch DOM Node property once.
+    return value == null ? super.toString() : value;
+  }
 $!MEMBERS
 }