Support all fast elements kinds in the major array operations.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index ab9ce1af127c07043a0909ec3908cb1b90a14696..f9f999059c8c35699739bf6bba7958fbb6b7200b 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1944,6 +1944,11 @@ void FixedArray::set_null_unchecked(Heap* heap, int index) {
 }
 
 
+double* FixedDoubleArray::data_start() {
+  return &READ_DOUBLE_FIELD(this, kHeaderSize);
+}
+
+
 Object** FixedArray::data_start() {
   return HeapObject::RawField(this, kHeaderSize);
 }
@@ -4814,6 +4819,11 @@ bool JSObject::HasFastHoleyElements() {
 }
 
 
+bool JSObject::HasFastElements() {
+  return IsFastElementsKind(GetElementsKind());
+}
+
+
 bool JSObject::HasDictionaryElements() {
   return GetElementsKind() == DICTIONARY_ELEMENTS;
 }