Tests for GuardField length check along with bug fixes

runtime/vm/code_generator.cc

Index: runtime/vm/code_generator.cc
diff --git a/runtime/vm/code_generator.cc b/runtime/vm/code_generator.cc
index a5a597f39afe165340ba84cfffeb10ae51696528..afde51241204cbc2d0d24a72b50f41812caf9ce1 100644
--- a/runtime/vm/code_generator.cc
+++ b/runtime/vm/code_generator.cc
@@ -1859,10 +1859,10 @@ static intptr_t GetListLength(const Object& value) {
   // Extract list length.
   if (value.IsTypedData()) {
     const TypedData& list = TypedData::Cast(value);
-    return list.Length();
+    return Smi::RawValue(list.Length());
   } else if (value.IsArray()) {
     const Array& list = Array::Cast(value);
-    return list.Length();
+    return Smi::RawValue(list.Length());
   } else if (value.IsGrowableObjectArray()) {
     // List length is variable.
     return Field::kNoFixedLength;