Disentangle assembler from isolate.

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 991a5dcacc49ca702efe49b67cd739ffed18e7c4..d3f25137994a2343b1d2d36a2c789ee251b973a0 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -485,7 +485,9 @@ class Assembler : public AssemblerBase {
   // for code generation and assumes its size to be buffer_size. If the buffer
   // is too small, a fatal error occurs. No deallocation of the buffer is done
   // upon destruction of the assembler.
-  Assembler(Isolate* isolate, void* buffer, int buffer_size);
+  Assembler(Isolate* isolate, void* buffer, int buffer_size)
+      : Assembler(IsolateData(isolate), buffer, buffer_size) {}
+  Assembler(IsolateData isolate_data, void* buffer, int buffer_size);
   virtual ~Assembler() { }
 
   // GetCode emits any pending (non-emitted) code and fills the descriptor
@@ -500,6 +502,7 @@ class Assembler : public AssemblerBase {
   // the absolute address of the target.
   // These functions convert between absolute Addresses of Code objects and
   // the relative displacements stored in the code.
+  // The isolate argument is unused (and may be nullptr) when skipping flushing.
   static inline Address target_address_at(Address pc, Address constant_pool);
   static inline void set_target_address_at(
       Isolate* isolate, Address pc, Address constant_pool, Address target,