Make FlushICache part of Assembler(Base) and take Isolate as parameter.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 47e4d2bddae310469a02e4a93c7e6681bbdad0ab..fd4e412e7a8e6d1356008dde84370626665aadad 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -507,6 +507,9 @@ class Assembler : public AssemblerBase {
   static const int kGap = 32;
 
  public:
+  using AssemblerBase::FlushICache;
+  using AssemblerBase::FlushICacheWithoutIsolate;
+
   // Create an assembler. Instructions and relocation information are emitted
   // into a buffer, with the instructions starting from the beginning and the
   // relocation information starting from the end of the buffer. See CodeDesc